<?xml version="1.0" encoding="utf-8"?>
<!--XBRL document created with Toppan Merrill Bridge  11.3.0.1 -->
<!--Based on XBRL 2.1-->
<!--Created on: 6/5/2026 7:22:10 PM-->
<!--Modified on: 6/5/2026 7:22:10 PM-->
<link:linkbase xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt">
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#StatementCondensedConsolidatedStatementsOfOperationsAndComprehensiveLoss" roleURI="http://www.terrestrialenergy.com/role/StatementCondensedConsolidatedStatementsOfOperationsAndComprehensiveLoss" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#StatementCondensedConsolidatedStatementsOfChangesInStockholdersEquityDeficit" roleURI="http://www.terrestrialenergy.com/role/StatementCondensedConsolidatedStatementsOfChangesInStockholdersEquityDeficit" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#StatementCondensedConsolidatedStatementsOfCashFlows" roleURI="http://www.terrestrialenergy.com/role/StatementCondensedConsolidatedStatementsOfCashFlows" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureFinancialInstrumentsInvestmentsBySignificantInvestmentCategoryDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureFinancialInstrumentsInvestmentsBySignificantInvestmentCategoryDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureRelatedPartyBalancesAndTransactionsDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureRelatedPartyBalancesAndTransactionsDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureStockholdersEquityDeficitDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureStockholdersEquityDeficitDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureNetLossPerShareOfCommonSharePotentiallyDilutiveSecuritiesExcludedFromCalculationDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureNetLossPerShareOfCommonSharePotentiallyDilutiveSecuritiesExcludedFromCalculationDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureStockBasedCompensationDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureStockBasedCompensationDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureSegmentInformationDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureSegmentInformationDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureSegmentInformationLongLivedAssetsDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureSegmentInformationLongLivedAssetsDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#StatementConsolidatedBalanceSheets" roleURI="http://www.terrestrialenergy.com/role/StatementConsolidatedBalanceSheets" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#StatementConsolidatedStatementsOfOperationsAndComprehensiveLoss" roleURI="http://www.terrestrialenergy.com/role/StatementConsolidatedStatementsOfOperationsAndComprehensiveLoss" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#StatementConsolidatedStatementsOfChangesInStockholdersEquityDeficit" roleURI="http://www.terrestrialenergy.com/role/StatementConsolidatedStatementsOfChangesInStockholdersEquityDeficit" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#StatementConsolidatedStatementsOfCashFlows" roleURI="http://www.terrestrialenergy.com/role/StatementConsolidatedStatementsOfCashFlows" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureConvertibleNotesAndConvertibleNotesRelatedPartiesTables" roleURI="http://www.terrestrialenergy.com/role/DisclosureConvertibleNotesAndConvertibleNotesRelatedPartiesTables" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureOrganizationAndDescriptionOfBusinessDetailss" roleURI="http://www.terrestrialenergy.com/role/DisclosureOrganizationAndDescriptionOfBusinessDetailss" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureSignificantAccountingPoliciesVariableInterestEntitiesDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureSignificantAccountingPoliciesVariableInterestEntitiesDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureSignificantAccountingPoliciesPropertyAndEquipmentNetDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureSignificantAccountingPoliciesPropertyAndEquipmentNetDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureReverseRecapitalizationDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureReverseRecapitalizationDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureReverseRecapitalizationReconciliationOfBusinessCombinationToCashFlowsDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureReverseRecapitalizationReconciliationOfBusinessCombinationToCashFlowsDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureReverseRecapitalizationCommonStockIssuedImmediatelyFollowingBusinessCombinationDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureReverseRecapitalizationCommonStockIssuedImmediatelyFollowingBusinessCombinationDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureReverseRecapitalizationTransactionExpensesDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureReverseRecapitalizationTransactionExpensesDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosurePropertyAndEquipmentNetDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosurePropertyAndEquipmentNetDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureIntangibleAssetsNetDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureIntangibleAssetsNetDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureFinancialInstrumentsDetailss" roleURI="http://www.terrestrialenergy.com/role/DisclosureFinancialInstrumentsDetailss" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureGovernmentGrantsDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureGovernmentGrantsDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureConvertibleNotesAndConvertibleNotesRelatedParties2024BridgeRoundOfferingDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureConvertibleNotesAndConvertibleNotesRelatedParties2024BridgeRoundOfferingDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureConvertibleNotesAndConvertibleNotesRelatedPartiesFebruary2025BridgeRoundOfferingDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureConvertibleNotesAndConvertibleNotesRelatedPartiesFebruary2025BridgeRoundOfferingDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureConvertibleNotesAndConvertibleNotesRelatedPartiesRollForwardOfConvertibleNotesAndConvertibleNotesRelatedPartyDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureConvertibleNotesAndConvertibleNotesRelatedPartiesRollForwardOfConvertibleNotesAndConvertibleNotesRelatedPartyDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureRelatedPartyBalancesAndTransactionsDetailss" roleURI="http://www.terrestrialenergy.com/role/DisclosureRelatedPartyBalancesAndTransactionsDetailss" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureStockholdersEquityDeficitDetailss" roleURI="http://www.terrestrialenergy.com/role/DisclosureStockholdersEquityDeficitDetailss" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureNetLossPerShareOfCommonSharePotentiallyDilutiveSecuritiesExcludedFromCalculationDetailss" roleURI="http://www.terrestrialenergy.com/role/DisclosureNetLossPerShareOfCommonSharePotentiallyDilutiveSecuritiesExcludedFromCalculationDetailss" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureStockBasedCompensationDetailss" roleURI="http://www.terrestrialenergy.com/role/DisclosureStockBasedCompensationDetailss" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureStockBasedCompensationStockBasedCompensationExpenseDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureStockBasedCompensationStockBasedCompensationExpenseDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureStockBasedCompensationRestrictedStockUnitDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureStockBasedCompensationRestrictedStockUnitDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureIncomeTaxesTaxCreditCarryforwardsDetails" roleURI="http://www.terrestrialenergy.com/role/DisclosureIncomeTaxesTaxCreditCarryforwardsDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureSegmentInformationDetailss" roleURI="http://www.terrestrialenergy.com/role/DisclosureSegmentInformationDetailss" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="tmb-20260331.xsd#DisclosureSegmentInformationLongLivedAssetsDetailss" roleURI="http://www.terrestrialenergy.com/role/DisclosureSegmentInformationLongLivedAssetsDetailss" xmlns="http://www.xbrl.org/2003/linkbase" />
  <link:roleRef roleURI="http://www.terrestrialenergy.com/role/DisclosureStandardAndCustomAxisDomainDefaults" xlink:href="tmb-20260331.xsd#DisclosureStandardAndCustomAxisDomainDefaults" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://www.esma.europa.eu/xbrl/esef/arcrole/wider-narrower" xlink:href="http://www.xbrl.org/lrr/arcrole/esma-arcrole-2018-11-21.xsd#wider-narrower" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.terrestrialenergy.com/role/DisclosureStandardAndCustomAxisDomainDefaults">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1" xlink:title="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain_1" xlink:title="us-gaap_RelatedPartyDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1" xlink:to="us-gaap_RelatedPartyDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_1 To us-gaap_RelatedPartyDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis_1" xlink:title="us-gaap_DebtInstrumentAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain_1" xlink:title="us-gaap_DebtInstrumentNameDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentAxis_1" xlink:to="us-gaap_DebtInstrumentNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_DebtInstrumentAxis_1 To us-gaap_DebtInstrumentNameDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis_1" xlink:title="us-gaap_StatementClassOfStockAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain_1" xlink:title="us-gaap_ClassOfStockDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_StatementClassOfStockAxis_1" xlink:to="us-gaap_ClassOfStockDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_StatementClassOfStockAxis_1 To us-gaap_ClassOfStockDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis_1" xlink:title="srt_RangeAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember_1" xlink:title="srt_RangeMember_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_RangeAxis_1" xlink:to="srt_RangeMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_RangeAxis_1 To srt_RangeMember_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis_1" xlink:title="us-gaap_StatementBusinessSegmentsAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain_1" xlink:title="us-gaap_SegmentDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_StatementBusinessSegmentsAxis_1" xlink:to="us-gaap_SegmentDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_StatementBusinessSegmentsAxis_1 To us-gaap_SegmentDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="us-gaap_SubsequentEventTypeAxis_1" xlink:title="us-gaap_SubsequentEventTypeAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="us-gaap_SubsequentEventTypeDomain_1" xlink:title="us-gaap_SubsequentEventTypeDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_SubsequentEventTypeAxis_1" xlink:to="us-gaap_SubsequentEventTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_SubsequentEventTypeAxis_1 To us-gaap_SubsequentEventTypeDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis" xlink:label="srt_ConsolidatedEntitiesAxis_1" xlink:title="srt_ConsolidatedEntitiesAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="srt_ConsolidatedEntitiesDomain_1" xlink:title="srt_ConsolidatedEntitiesDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_ConsolidatedEntitiesAxis_1" xlink:to="srt_ConsolidatedEntitiesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_ConsolidatedEntitiesAxis_1 To srt_ConsolidatedEntitiesDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipAxis" xlink:label="srt_OwnershipAxis_1" xlink:title="srt_OwnershipAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain" xlink:label="srt_OwnershipDomain_1" xlink:title="srt_OwnershipDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_OwnershipAxis_1" xlink:to="srt_OwnershipDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_OwnershipAxis_1 To srt_OwnershipDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis_1" xlink:title="dei_LegalEntityAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain_1" xlink:title="dei_EntityDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="dei_LegalEntityAxis_1" xlink:to="dei_EntityDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:dei_LegalEntityAxis_1 To dei_EntityDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementScenarioAxis" xlink:label="srt_StatementScenarioAxis_1" xlink:title="srt_StatementScenarioAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="srt_ScenarioUnspecifiedDomain_1" xlink:title="srt_ScenarioUnspecifiedDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_StatementScenarioAxis_1" xlink:to="srt_ScenarioUnspecifiedDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_StatementScenarioAxis_1 To srt_ScenarioUnspecifiedDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeAxis" xlink:label="us-gaap_ShortTermDebtTypeAxis_1" xlink:title="us-gaap_ShortTermDebtTypeAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermDebtTypeDomain" xlink:label="us-gaap_ShortTermDebtTypeDomain_1" xlink:title="us-gaap_ShortTermDebtTypeDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ShortTermDebtTypeAxis_1" xlink:to="us-gaap_ShortTermDebtTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_ShortTermDebtTypeAxis_1 To us-gaap_ShortTermDebtTypeDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeAxis" xlink:label="us-gaap_MeasurementInputTypeAxis_1" xlink:title="us-gaap_MeasurementInputTypeAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="us-gaap_MeasurementInputTypeDomain_1" xlink:title="us-gaap_MeasurementInputTypeDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_MeasurementInputTypeAxis_1" xlink:to="us-gaap_MeasurementInputTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_MeasurementInputTypeAxis_1 To us-gaap_MeasurementInputTypeDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis_1" xlink:title="us-gaap_StatementEquityComponentsAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain_1" xlink:title="us-gaap_EquityComponentDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_StatementEquityComponentsAxis_1" xlink:to="us-gaap_EquityComponentDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_StatementEquityComponentsAxis_1 To us-gaap_EquityComponentDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis_1" xlink:title="us-gaap_PropertyPlantAndEquipmentByTypeAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain_1" xlink:title="us-gaap_PropertyPlantAndEquipmentTypeDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis_1" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_PropertyPlantAndEquipmentByTypeAxis_1 To us-gaap_PropertyPlantAndEquipmentTypeDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:label="us-gaap_ConcentrationRiskByBenchmarkAxis_1" xlink:title="us-gaap_ConcentrationRiskByBenchmarkAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="us-gaap_ConcentrationRiskBenchmarkDomain_1" xlink:title="us-gaap_ConcentrationRiskBenchmarkDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ConcentrationRiskByBenchmarkAxis_1" xlink:to="us-gaap_ConcentrationRiskBenchmarkDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_ConcentrationRiskByBenchmarkAxis_1 To us-gaap_ConcentrationRiskBenchmarkDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis" xlink:label="us-gaap_ConcentrationRiskByTypeAxis_1" xlink:title="us-gaap_ConcentrationRiskByTypeAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="us-gaap_ConcentrationRiskTypeDomain_1" xlink:title="us-gaap_ConcentrationRiskTypeDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ConcentrationRiskByTypeAxis_1" xlink:to="us-gaap_ConcentrationRiskTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_ConcentrationRiskByTypeAxis_1 To us-gaap_ConcentrationRiskTypeDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1" xlink:title="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1" xlink:title="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_1 To us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="us-gaap_IncomeTaxAuthorityAxis_1" xlink:title="us-gaap_IncomeTaxAuthorityAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain_1" xlink:title="us-gaap_IncomeTaxAuthorityDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_IncomeTaxAuthorityAxis_1" xlink:to="us-gaap_IncomeTaxAuthorityDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_IncomeTaxAuthorityAxis_1 To us-gaap_IncomeTaxAuthorityDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis_1" xlink:title="us-gaap_SubsidiarySaleOfStockAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_1" xlink:title="us-gaap_SaleOfStockNameOfTransactionDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_SubsidiarySaleOfStockAxis_1" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_SubsidiarySaleOfStockAxis_1 To us-gaap_SaleOfStockNameOfTransactionDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="srt_CounterpartyNameAxis_1" xlink:title="srt_CounterpartyNameAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain_1" xlink:title="srt_RepurchaseAgreementCounterpartyNameDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_CounterpartyNameAxis_1" xlink:to="srt_RepurchaseAgreementCounterpartyNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_CounterpartyNameAxis_1 To srt_RepurchaseAgreementCounterpartyNameDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis_1" xlink:title="us-gaap_BusinessAcquisitionAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain_1" xlink:title="us-gaap_BusinessAcquisitionAcquireeDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_BusinessAcquisitionAxis_1" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_BusinessAcquisitionAxis_1 To us-gaap_BusinessAcquisitionAcquireeDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAxis" xlink:label="us-gaap_CashAndCashEquivalentsAxis_1" xlink:title="us-gaap_CashAndCashEquivalentsAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_1" xlink:title="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_CashAndCashEquivalentsAxis_1" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_CashAndCashEquivalentsAxis_1 To us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis_1" xlink:title="us-gaap_FairValueByFairValueHierarchyLevelAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" xlink:title="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis_1" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_FairValueByFairValueHierarchyLevelAxis_1 To us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis_1" xlink:title="us-gaap_AwardTypeAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" xlink:title="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_AwardTypeAxis_1" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_AwardTypeAxis_1 To us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementAxis" xlink:label="srt_RestatementAxis_1" xlink:title="srt_RestatementAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementDomain" xlink:label="srt_RestatementDomain_1" xlink:title="srt_RestatementDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_RestatementAxis_1" xlink:to="srt_RestatementDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_RestatementAxis_1 To srt_RestatementDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_1" xlink:title="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain_1" xlink:title="us-gaap_AntidilutiveSecuritiesNameDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_1" xlink:to="us-gaap_AntidilutiveSecuritiesNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_1 To us-gaap_AntidilutiveSecuritiesNameDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis" xlink:label="srt_StatementGeographicalAxis_1" xlink:title="srt_StatementGeographicalAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="srt_SegmentGeographicalDomain_1" xlink:title="srt_SegmentGeographicalDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_StatementGeographicalAxis_1" xlink:to="srt_SegmentGeographicalDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_StatementGeographicalAxis_1 To srt_SegmentGeographicalDomain_1" order="1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis_1" xlink:title="us-gaap_FinancialInstrumentAxis_1" xlink:type="locator" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1" xlink:title="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1" xlink:type="locator" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_FinancialInstrumentAxis_1" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:us-gaap_FinancialInstrumentAxis_1 To us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1" order="1" />
  </link:definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/StatementCondensedConsolidatedStatementsOfOperationsAndComprehensiveLoss" xlink:type="extended" xlink:title="100200 - Statement - Condensed Consolidated Statements of Operations and Comprehensive Loss" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonrelatedPartyMember" xlink:label="us-gaap_NonrelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_NonrelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="us-gaap_RelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_RelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpensesAbstract" xlink:label="us-gaap_OperatingExpensesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingExpensesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="us-gaap_ResearchAndDevelopmentExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_ResearchAndDevelopmentExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpense" xlink:label="us-gaap_GeneralAndAdministrativeExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_GeneralAndAdministrativeExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="us-gaap_DepreciationDepletionAndAmortization" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_DepreciationDepletionAndAmortization" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpenses" xlink:label="us-gaap_OperatingExpenses" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_OperatingExpenses" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingIncomeLoss" xlink:label="us-gaap_OperatingIncomeLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingIncomeLoss" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonoperatingIncomeExpenseAbstract" xlink:label="us-gaap_NonoperatingIncomeExpenseAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NonoperatingIncomeExpenseAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceNonoperatingIncome" xlink:label="us-gaap_GovernmentAssistanceNonoperatingIncome" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_GovernmentAssistanceNonoperatingIncome" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating" xlink:label="us-gaap_InterestExpenseNonoperating" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_InterestExpenseNonoperating" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeInterestAndDividend" xlink:label="us-gaap_InvestmentIncomeInterestAndDividend" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_InvestmentIncomeInterestAndDividend" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyTransactionGainLossBeforeTax" xlink:label="us-gaap_ForeignCurrencyTransactionGainLossBeforeTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_ForeignCurrencyTransactionGainLossBeforeTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonoperatingIncomeExpense" xlink:label="us-gaap_NonoperatingIncomeExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_NonoperatingIncomeExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:label="us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="us-gaap_IncomeTaxExpenseBenefit" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxExpenseBenefit" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLoss" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic" xlink:label="us-gaap_EarningsPerShareBasic" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EarningsPerShareBasic" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="us-gaap_EarningsPerShareDiluted" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EarningsPerShareDiluted" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent" xlink:label="us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent" xlink:label="us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" xlink:label="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/StatementCondensedConsolidatedStatementsOfChangesInStockholdersEquityDeficit" xlink:type="extended" xlink:title="100300 - Statement - Condensed Consolidated Statements of Changes in Stockholders' Equity (Deficit)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementEquityComponentsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ParentMember" xlink:label="us-gaap_ParentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_ParentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember" xlink:label="us-gaap_PreferredStockMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_PreferredStockMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_CommonStockMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ExchangeableStockMember" xlink:label="imsr_ExchangeableStockMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ParentMember" xlink:to="imsr_ExchangeableStockMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="us-gaap_AdditionalPaidInCapitalMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_AdditionalPaidInCapitalMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember" xlink:label="us-gaap_RetainedEarningsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_RetainedEarningsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementAxis" xlink:label="srt_RestatementAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_StatementTable" xlink:to="srt_RestatementAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementDomain" xlink:label="srt_RestatementDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RestatementAxis" xlink:to="srt_RestatementDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioPreviouslyReportedMember" xlink:label="srt_ScenarioPreviouslyReportedMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RestatementDomain" xlink:to="srt_ScenarioPreviouslyReportedMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementAdjustmentMember" xlink:label="srt_RestatementAdjustmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_RestatementDomain" xlink:to="srt_RestatementAdjustmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:label="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesIssued" xlink:label="us-gaap_SharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_SharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_NumberOfSharesExchangedDuringPeriod" xlink:label="imsr_NumberOfSharesExchangedDuringPeriod" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="imsr_NumberOfSharesExchangedDuringPeriod" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodValueNewIssues" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent" xlink:label="us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentNetOfTaxPortionAttributableToParent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent" xlink:label="us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="us-gaap_ProfitLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_ProfitLoss" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/StatementCondensedConsolidatedStatementsOfCashFlows" xlink:type="extended" xlink:title="100400 - Statement - Condensed Consolidated Statements of Cash Flows" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonrelatedPartyMember" xlink:label="us-gaap_NonrelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_NonrelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="us-gaap_RelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_RelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="us-gaap_ProfitLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_ProfitLoss" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="us-gaap_DepreciationDepletionAndAmortization" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_DepreciationDepletionAndAmortization" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts" xlink:label="us-gaap_AmortizationOfFinancingCosts" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_AmortizationOfFinancingCosts" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeNet" xlink:label="us-gaap_InvestmentIncomeNet" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_InvestmentIncomeNet" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation" xlink:label="us-gaap_ShareBasedCompensation" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_ShareBasedCompensation" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyTransactionGainLossUnrealized" xlink:label="us-gaap_ForeignCurrencyTransactionGainLossUnrealized" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_ForeignCurrencyTransactionGainLossUnrealized" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense" xlink:label="us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:label="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_IncreaseDecreaseInInterestPayableNonCurrent" xlink:label="imsr_IncreaseDecreaseInInterestPayableNonCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="imsr_IncreaseDecreaseInInterestPayableNonCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingLeaseLiability" xlink:label="us-gaap_IncreaseDecreaseInOperatingLeaseLiability" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInOperatingLeaseLiability" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireIntangibleAssets" xlink:label="us-gaap_PaymentsToAcquireIntangibleAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireIntangibleAssets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" xlink:label="us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt" xlink:label="us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities" xlink:label="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromNotesPayable" xlink:label="us-gaap_ProceedsFromNotesPayable" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromNotesPayable" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromStockOptionsExercised" xlink:label="us-gaap_ProceedsFromStockOptionsExercised" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromStockOptionsExercised" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeasePrincipalPayments" xlink:label="us-gaap_FinanceLeasePrincipalPayments" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_FinanceLeasePrincipalPayments" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" xlink:label="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_RecognitionOfWarrantsInConnectionWithConvertibleNotesNetOfTax" xlink:label="imsr_RecognitionOfWarrantsInConnectionWithConvertibleNotesNetOfTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" xlink:to="imsr_RecognitionOfWarrantsInConnectionWithConvertibleNotesNetOfTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_OperatingLeaseLiabilityObtainedInExchangeForRightOfUseAsset" xlink:label="imsr_OperatingLeaseLiabilityObtainedInExchangeForRightOfUseAsset" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" xlink:to="imsr_OperatingLeaseLiabilityObtainedInExchangeForRightOfUseAsset" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureFinancialInstrumentsInvestmentsBySignificantInvestmentCategoryDetails" xlink:type="extended" xlink:title="140301 - Disclosure - Financial Instruments - Investments by significant investment category (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByBalanceSheetGroupingTable" xlink:label="us-gaap_FairValueByBalanceSheetGroupingTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_FairValueByBalanceSheetGroupingTable" xlink:to="us-gaap_FinancialInstrumentAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashMember" xlink:label="us-gaap_CashMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_CashMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsMember" xlink:label="us-gaap_MoneyMarketFundsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_MoneyMarketFundsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_FairValueByBalanceSheetGroupingTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="us-gaap_FairValueInputsLevel1Member" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel1Member" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="us-gaap_FairValueInputsLevel2Member" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel2Member" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:label="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_FairValueByBalanceSheetGroupingTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossCurrent" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTaxCurrent" xlink:label="imsr_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTaxCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="imsr_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainBeforeTaxCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossNoncurrent" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossNoncurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLossNoncurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTaxNoncurrent" xlink:label="imsr_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTaxNoncurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="imsr_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossBeforeTaxNoncurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure" xlink:label="us-gaap_CashAndCashEquivalentsFairValueDisclosure" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_CashAndCashEquivalentsFairValueDisclosure" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterestCurrent" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterestCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterestCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterestNoncurrent" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterestNoncurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems" xlink:to="us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterestNoncurrent" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureRelatedPartyBalancesAndTransactionsDetails" xlink:type="extended" xlink:title="140401 - Disclosure - Related Party Balances and Transactions (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="us-gaap_RelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_RelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_DebtInstrumentAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_February2025BridgeRoundOfferingMember" xlink:label="imsr_February2025BridgeRoundOfferingMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="imsr_February2025BridgeRoundOfferingMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="srt_CounterpartyNameAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="srt_CounterpartyNameAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_CounterpartyNameAxis" xlink:to="srt_RepurchaseAgreementCounterpartyNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_SwhCapitalLlcMember" xlink:label="imsr_SwhCapitalLlcMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RepurchaseAgreementCounterpartyNameDomain" xlink:to="imsr_SwhCapitalLlcMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="us-gaap_RelatedPartyTransactionLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfessionalFees" xlink:label="us-gaap_ProfessionalFees" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ProfessionalFees" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="us-gaap_ResearchAndDevelopmentExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ResearchAndDevelopmentExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent" xlink:label="us-gaap_AccountsPayableCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_AccountsPayableCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions" xlink:label="us-gaap_StockIssuedDuringPeriodSharesAcquisitions" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesAcquisitions" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromRelatedPartyDebt" xlink:label="us-gaap_ProceedsFromRelatedPartyDebt" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ProceedsFromRelatedPartyDebt" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureStockholdersEquityDeficitDetails" xlink:type="extended" xlink:title="140501 - Disclosure - Stockholders' Equity (Deficit) (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="us-gaap_ScheduleOfStockByClassTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="us-gaap_StatementClassOfStockAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="us-gaap_SeriesAPreferredStockMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesAPreferredStockMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="srt_RangeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems" xlink:label="us-gaap_ClassOfStockLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ScheduleOfStockByClassTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:label="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="us-gaap_PreferredStockSharesAuthorized" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_PreferredStockSharesAuthorized" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_PreferredStockParOrStatedValuePerShare" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="us-gaap_PreferredStockSharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_PreferredStockSharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="us-gaap_PreferredStockSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_PreferredStockSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_NumberOfAdditionalSharesIssuedAndConvertedOfPreferredStock" xlink:label="imsr_NumberOfAdditionalSharesIssuedAndConvertedOfPreferredStock" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="imsr_NumberOfAdditionalSharesIssuedAndConvertedOfPreferredStock" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_StockIssuedDuringPeriodSharesConversionOfPreferredStock" xlink:label="imsr_StockIssuedDuringPeriodSharesConversionOfPreferredStock" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="imsr_StockIssuedDuringPeriodSharesConversionOfPreferredStock" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:label="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="us-gaap_CommonStockSharesAuthorized" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_CommonStockSharesAuthorized" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_CommonStockNumberOfVotesPerShare" xlink:label="imsr_CommonStockNumberOfVotesPerShare" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="imsr_CommonStockNumberOfVotesPerShare" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued" xlink:label="us-gaap_CommonStockSharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_CommonStockSharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="us-gaap_CommonStockSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_CommonStockSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsNoteDisclosureAbstract" xlink:label="us-gaap_WarrantsAndRightsNoteDisclosureAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_WarrantsAndRightsNoteDisclosureAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightOutstanding" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_WarrantsAndRightsNoteDisclosureAbstract" xlink:to="us-gaap_ClassOfWarrantOrRightOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_WarrantsAndRightsNoteDisclosureAbstract" xlink:to="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased" xlink:label="srt_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="srt_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ShareRepurchaseProgramExercisePrice" xlink:label="imsr_ShareRepurchaseProgramExercisePrice" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_ShareRepurchaseProgramExercisePrice" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ExchangeableStockSharesOutstanding" xlink:label="imsr_ExchangeableStockSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_ExchangeableStockSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_CommonExchangeableStockSharesOutstanding" xlink:label="imsr_CommonExchangeableStockSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_CommonExchangeableStockSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_PreferredExchangeableStockSharesOutstanding" xlink:label="imsr_PreferredExchangeableStockSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_PreferredExchangeableStockSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ExchangeableSharesConversionRatio" xlink:label="imsr_ExchangeableSharesConversionRatio" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_ExchangeableSharesConversionRatio" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_NumberOfSharesExchangedDuringPeriod" xlink:label="imsr_NumberOfSharesExchangedDuringPeriod" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_NumberOfSharesExchangedDuringPeriod" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureNetLossPerShareOfCommonSharePotentiallyDilutiveSecuritiesExcludedFromCalculationDetails" xlink:type="extended" xlink:title="140603 - Disclosure - Net Loss Per Share of Common Share - Potentially dilutive securities excluded from the calculation (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:to="us-gaap_AntidilutiveSecuritiesNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AntidilutiveSecuritiesNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="us-gaap_RestrictedStockUnitsRSUMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AntidilutiveSecuritiesNameDomain" xlink:to="us-gaap_RestrictedStockUnitsRSUMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantMember" xlink:label="us-gaap_WarrantMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_AntidilutiveSecuritiesNameDomain" xlink:to="us-gaap_WarrantMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:to="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureStockBasedCompensationDetails" xlink:type="extended" xlink:title="140701 - Disclosure - Stock-Based Compensation (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="us-gaap_RestrictedStockUnitsRSUMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_RestrictedStockUnitsRSUMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="srt_RangeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureSegmentInformationDetails" xlink:type="extended" xlink:title="140901 - Disclosure - Segment Information (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_SingleReportableSegmentMember" xlink:label="imsr_SingleReportableSegmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentDomain" xlink:to="imsr_SingleReportableSegmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="us-gaap_SegmentReportingInformationLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfOperatingSegments" xlink:label="us-gaap_NumberOfOperatingSegments" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_NumberOfOperatingSegments" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfReportableSegments" xlink:label="us-gaap_NumberOfReportableSegments" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_NumberOfReportableSegments" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="us-gaap_ResearchAndDevelopmentExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ResearchAndDevelopmentExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpense" xlink:label="us-gaap_GeneralAndAdministrativeExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_GeneralAndAdministrativeExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="us-gaap_DepreciationDepletionAndAmortization" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_DepreciationDepletionAndAmortization" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpenses" xlink:label="us-gaap_OperatingExpenses" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_OperatingExpenses" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureSegmentInformationLongLivedAssetsDetails" xlink:type="extended" xlink:title="140902 - Disclosure - Segment Information - Long-lived assets (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable" xlink:label="us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis" xlink:label="srt_StatementGeographicalAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable" xlink:to="srt_StatementGeographicalAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="srt_SegmentGeographicalDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_StatementGeographicalAxis" xlink:to="srt_SegmentGeographicalDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US" xlink:label="country_US" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_SegmentGeographicalDomain" xlink:to="country_US" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CA" xlink:label="country_CA" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_SegmentGeographicalDomain" xlink:to="country_CA" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems" xlink:label="us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems" xlink:to="us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncurrentAssets" xlink:label="us-gaap_NoncurrentAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems" xlink:to="us-gaap_NoncurrentAssets" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/StatementConsolidatedBalanceSheets" xlink:type="extended" xlink:title="200100 - Statement - Consolidated Balance Sheets" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementAxis" xlink:label="srt_RestatementAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="srt_RestatementAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementDomain" xlink:label="srt_RestatementDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RestatementAxis" xlink:to="srt_RestatementDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioPreviouslyReportedMember" xlink:label="srt_ScenarioPreviouslyReportedMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RestatementDomain" xlink:to="srt_ScenarioPreviouslyReportedMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonrelatedPartyMember" xlink:label="us-gaap_NonrelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_NonrelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="us-gaap_RelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_RelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract" xlink:label="us-gaap_AssetsAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AssetsAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrentAbstract" xlink:label="us-gaap_AssetsCurrentAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_AssetsCurrentAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_CashAndCashEquivalentsAtCarryingValue" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermInvestments" xlink:label="us-gaap_ShortTermInvestments" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_ShortTermInvestments" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_PrepaidExpenseAndOtherAssetsCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrent" xlink:label="us-gaap_AssetsCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_AssetsCurrentAbstract" xlink:to="us-gaap_AssetsCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="us-gaap_IntangibleAssetsNetExcludingGoodwill" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_IntangibleAssetsNetExcludingGoodwill" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset" xlink:label="us-gaap_OperatingLeaseRightOfUseAsset" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_OperatingLeaseRightOfUseAsset" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="us-gaap_OtherAssetsNoncurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_OtherAssetsNoncurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets" xlink:label="us-gaap_Assets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_Assets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LiabilitiesAndStockholdersEquityAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:label="us-gaap_LiabilitiesCurrentAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_LiabilitiesCurrentAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent" xlink:label="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityCurrent" xlink:label="us-gaap_OperatingLeaseLiabilityCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_OperatingLeaseLiabilityCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeaseLiabilityCurrent" xlink:label="us-gaap_FinanceLeaseLiabilityCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_FinanceLeaseLiabilityCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDepositLiabilities" xlink:label="us-gaap_RelatedPartyDepositLiabilities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_RelatedPartyDepositLiabilities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrent" xlink:label="us-gaap_LiabilitiesCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_LiabilitiesCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleLongTermNotesPayable" xlink:label="us-gaap_ConvertibleLongTermNotesPayable" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_ConvertibleLongTermNotesPayable" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_InterestPayableNoncurrent" xlink:label="imsr_InterestPayableNoncurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="imsr_InterestPayableNoncurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent" xlink:label="us-gaap_OperatingLeaseLiabilityNoncurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_OperatingLeaseLiabilityNoncurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeaseLiabilityNoncurrent" xlink:label="us-gaap_FinanceLeaseLiabilityNoncurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_FinanceLeaseLiabilityNoncurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet" xlink:label="us-gaap_DeferredIncomeTaxLiabilitiesNet" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_DeferredIncomeTaxLiabilitiesNet" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Liabilities" xlink:label="us-gaap_Liabilities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_Liabilities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsAndContingencies" xlink:label="us-gaap_CommitmentsAndContingencies" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_CommitmentsAndContingencies" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="us-gaap_StockholdersEquityAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_StockholdersEquityAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockValue" xlink:label="us-gaap_CommonStockValue" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_CommonStockValue" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ExchangeableStockValueIssued" xlink:label="imsr_ExchangeableStockValueIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="imsr_ExchangeableStockValueIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapital" xlink:label="us-gaap_AdditionalPaidInCapital" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_AdditionalPaidInCapital" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_RetainedEarningsAccumulatedDeficit" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_LiabilitiesAndStockholdersEquity" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/StatementConsolidatedStatementsOfOperationsAndComprehensiveLoss" xlink:type="extended" xlink:title="200200 - Statement - Consolidated Statements of Operations and Comprehensive Loss" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonrelatedPartyMember" xlink:label="us-gaap_NonrelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_NonrelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="us-gaap_RelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_RelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesAbstract" xlink:label="us-gaap_RevenuesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_RevenuesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerProductAndServiceExtensibleList" xlink:label="us-gaap_RevenueFromContractWithCustomerProductAndServiceExtensibleList" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_RevenueFromContractWithCustomerProductAndServiceExtensibleList" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpensesAbstract" xlink:label="us-gaap_OperatingExpensesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingExpensesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="us-gaap_ResearchAndDevelopmentExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_ResearchAndDevelopmentExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpense" xlink:label="us-gaap_GeneralAndAdministrativeExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_GeneralAndAdministrativeExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="us-gaap_DepreciationDepletionAndAmortization" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_DepreciationDepletionAndAmortization" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpenses" xlink:label="us-gaap_OperatingExpenses" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_OperatingExpenses" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingIncomeLoss" xlink:label="us-gaap_OperatingIncomeLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingIncomeLoss" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonoperatingIncomeExpenseAbstract" xlink:label="us-gaap_NonoperatingIncomeExpenseAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NonoperatingIncomeExpenseAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceNonoperatingIncome" xlink:label="us-gaap_GovernmentAssistanceNonoperatingIncome" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_GovernmentAssistanceNonoperatingIncome" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating" xlink:label="us-gaap_InterestExpenseNonoperating" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_InterestExpenseNonoperating" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:label="us-gaap_GainsLossesOnExtinguishmentOfDebt" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_GainsLossesOnExtinguishmentOfDebt" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeInterestAndDividend" xlink:label="us-gaap_InvestmentIncomeInterestAndDividend" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_InvestmentIncomeInterestAndDividend" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyTransactionGainLossBeforeTax" xlink:label="us-gaap_ForeignCurrencyTransactionGainLossBeforeTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_ForeignCurrencyTransactionGainLossBeforeTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonoperatingIncomeExpense" xlink:label="us-gaap_NonoperatingIncomeExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_NonoperatingIncomeExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" xlink:label="us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="us-gaap_IncomeTaxExpenseBenefit" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxExpenseBenefit" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLoss" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic" xlink:label="us-gaap_EarningsPerShareBasic" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EarningsPerShareBasic" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="us-gaap_EarningsPerShareDiluted" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EarningsPerShareDiluted" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" xlink:label="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:to="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/StatementConsolidatedStatementsOfChangesInStockholdersEquityDeficit" xlink:type="extended" xlink:title="200300 - Statement - Consolidated Statements of Changes in Stockholders' Equity (Deficit)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementEquityComponentsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ParentMember" xlink:label="us-gaap_ParentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_ParentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember" xlink:label="us-gaap_PreferredStockMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_PreferredStockMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_CommonStockMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ExchangeableStockMember" xlink:label="imsr_ExchangeableStockMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ParentMember" xlink:to="imsr_ExchangeableStockMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_PreferredExchangeableStockMember" xlink:label="imsr_PreferredExchangeableStockMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ParentMember" xlink:to="imsr_PreferredExchangeableStockMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_CommonExchangeableStockMember" xlink:label="imsr_CommonExchangeableStockMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ParentMember" xlink:to="imsr_CommonExchangeableStockMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="us-gaap_AdditionalPaidInCapitalMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_AdditionalPaidInCapitalMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember" xlink:label="us-gaap_RetainedEarningsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_ParentMember" xlink:to="us-gaap_RetainedEarningsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncontrollingInterestMember" xlink:label="us-gaap_NoncontrollingInterestMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_NoncontrollingInterestMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementAxis" xlink:label="srt_RestatementAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_StatementTable" xlink:to="srt_RestatementAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementDomain" xlink:label="srt_RestatementDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RestatementAxis" xlink:to="srt_RestatementDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioPreviouslyReportedMember" xlink:label="srt_ScenarioPreviouslyReportedMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RestatementDomain" xlink:to="srt_ScenarioPreviouslyReportedMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementAdjustmentMember" xlink:label="srt_RestatementAdjustmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_RestatementDomain" xlink:to="srt_RestatementAdjustmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_CashIssuanceMember" xlink:label="imsr_CashIssuanceMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="imsr_CashIssuanceMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrivatePlacementMember" xlink:label="us-gaap_PrivatePlacementMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_PrivatePlacementMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:label="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesIssued" xlink:label="us-gaap_SharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_SharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueAcquisitions" xlink:label="us-gaap_StockIssuedDuringPeriodValueAcquisitions" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodValueAcquisitions" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions" xlink:label="us-gaap_StockIssuedDuringPeriodSharesAcquisitions" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodSharesAcquisitions" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodValueNewIssues" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_AdjustmentsToAdditionalPaidInCapitalGainLossOnExtinguishmentOfDebt" xlink:label="imsr_AdjustmentsToAdditionalPaidInCapitalGainLossOnExtinguishmentOfDebt" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="imsr_AdjustmentsToAdditionalPaidInCapitalGainLossOnExtinguishmentOfDebt" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_StockIssuedDuringPeriodSharesConversionOfPreferredStock" xlink:label="imsr_StockIssuedDuringPeriodSharesConversionOfPreferredStock" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="imsr_StockIssuedDuringPeriodSharesConversionOfPreferredStock" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_StockIssuedDuringPeriodValueWarrantExercises" xlink:label="imsr_StockIssuedDuringPeriodValueWarrantExercises" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="imsr_StockIssuedDuringPeriodValueWarrantExercises" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_StockIssuedDuringPeriodSharesWarrantExercises" xlink:label="imsr_StockIssuedDuringPeriodSharesWarrantExercises" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="imsr_StockIssuedDuringPeriodSharesWarrantExercises" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="14" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_StockIssuedDuringPeriodValueMergerFinancing" xlink:label="imsr_StockIssuedDuringPeriodValueMergerFinancing" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="15" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="imsr_StockIssuedDuringPeriodValueMergerFinancing" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_StockIssuedDuringPeriodSharesMergerFinancing" xlink:label="imsr_StockIssuedDuringPeriodSharesMergerFinancing" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="16" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="imsr_StockIssuedDuringPeriodSharesMergerFinancing" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_AdjustmentsToAdditionalPaidInCapitalStockIssuedMergerAndRecapitalizationTransactionCosts" xlink:label="imsr_AdjustmentsToAdditionalPaidInCapitalStockIssuedMergerAndRecapitalizationTransactionCosts" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="17" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="imsr_AdjustmentsToAdditionalPaidInCapitalStockIssuedMergerAndRecapitalizationTransactionCosts" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities" xlink:label="us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="18" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" xlink:label="us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="19" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="20" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="21" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="us-gaap_ProfitLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="22" xlink:from="us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:to="us-gaap_ProfitLoss" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/StatementConsolidatedStatementsOfCashFlows" xlink:type="extended" xlink:title="200400 - Statement - Consolidated Statements of Cash Flows" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonrelatedPartyMember" xlink:label="us-gaap_NonrelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_NonrelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="us-gaap_RelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_RelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="us-gaap_ProfitLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_ProfitLoss" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="us-gaap_DepreciationDepletionAndAmortization" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_DepreciationDepletionAndAmortization" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:label="us-gaap_GainsLossesOnExtinguishmentOfDebt" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_GainsLossesOnExtinguishmentOfDebt" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts" xlink:label="us-gaap_AmortizationOfFinancingCosts" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_AmortizationOfFinancingCosts" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation" xlink:label="us-gaap_ShareBasedCompensation" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_ShareBasedCompensation" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyTransactionGainLossUnrealized" xlink:label="us-gaap_ForeignCurrencyTransactionGainLossUnrealized" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_ForeignCurrencyTransactionGainLossUnrealized" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense" xlink:label="us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxExpenseBenefit" xlink:label="us-gaap_DeferredIncomeTaxExpenseBenefit" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_DeferredIncomeTaxExpenseBenefit" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable" xlink:label="us-gaap_IncreaseDecreaseInAccountsReceivable" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInAccountsReceivable" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:label="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_IncreaseDecreaseInInterestPayableNonCurrent" xlink:label="imsr_IncreaseDecreaseInInterestPayableNonCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="imsr_IncreaseDecreaseInInterestPayableNonCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingLeaseLiability" xlink:label="us-gaap_IncreaseDecreaseInOperatingLeaseLiability" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInOperatingLeaseLiability" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireIntangibleAssets" xlink:label="us-gaap_PaymentsToAcquireIntangibleAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireIntangibleAssets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" xlink:label="us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireShortTermInvestments" xlink:label="us-gaap_PaymentsToAcquireShortTermInvestments" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireShortTermInvestments" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromNotesPayable" xlink:label="us-gaap_ProceedsFromNotesPayable" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromNotesPayable" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" xlink:label="us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromIssuanceOfPreferredStockAndPreferenceStock" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromStockOptionsExercised" xlink:label="us-gaap_ProceedsFromStockOptionsExercised" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromStockOptionsExercised" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromWarrantExercises" xlink:label="us-gaap_ProceedsFromWarrantExercises" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromWarrantExercises" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement" xlink:label="us-gaap_ProceedsFromIssuanceOfPrivatePlacement" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromIssuanceOfPrivatePlacement" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ProceedsFromMergerFinancing" xlink:label="imsr_ProceedsFromMergerFinancing" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="imsr_ProceedsFromMergerFinancing" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_PaymentOfMergerAndRecapitalizationRelatedTransactionCosts" xlink:label="imsr_PaymentOfMergerAndRecapitalizationRelatedTransactionCosts" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="imsr_PaymentOfMergerAndRecapitalizationRelatedTransactionCosts" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromRelatedPartyDebt" xlink:label="us-gaap_ProceedsFromRelatedPartyDebt" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromRelatedPartyDebt" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeasePrincipalPayments" xlink:label="us-gaap_FinanceLeasePrincipalPayments" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_FinanceLeasePrincipalPayments" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SupplementalCashFlowInformationAbstract" xlink:label="us-gaap_SupplementalCashFlowInformationAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SupplementalCashFlowInformationAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPaidNet" xlink:label="us-gaap_InterestPaidNet" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SupplementalCashFlowInformationAbstract" xlink:to="us-gaap_InterestPaidNet" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" xlink:label="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtConversionConvertedInstrumentAmount1" xlink:label="us-gaap_DebtConversionConvertedInstrumentAmount1" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" xlink:to="us-gaap_DebtConversionConvertedInstrumentAmount1" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_FinanceLeasesRecognized" xlink:label="imsr_FinanceLeasesRecognized" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" xlink:to="imsr_FinanceLeasesRecognized" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_RecognitionOfWarrantsInConnectionWithConvertibleNotesNetOfTax" xlink:label="imsr_RecognitionOfWarrantsInConnectionWithConvertibleNotesNetOfTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" xlink:to="imsr_RecognitionOfWarrantsInConnectionWithConvertibleNotesNetOfTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ExtinguishmentOfDebtAmountRelatedParty" xlink:label="imsr_ExtinguishmentOfDebtAmountRelatedParty" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" xlink:to="imsr_ExtinguishmentOfDebtAmountRelatedParty" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_AcquisitionOfNonControllingInterest" xlink:label="imsr_AcquisitionOfNonControllingInterest" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" xlink:to="imsr_AcquisitionOfNonControllingInterest" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureConvertibleNotesAndConvertibleNotesRelatedPartiesTables" xlink:type="extended" xlink:title="231103 - Disclosure - Convertible Notes and Convertible Notes - Related Parties (Tables)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_DebtInstrumentAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_BridgeRoundOffering2024Member" xlink:label="imsr_BridgeRoundOffering2024Member" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="imsr_BridgeRoundOffering2024Member" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_February2025BridgeRoundOfferingMember" xlink:label="imsr_February2025BridgeRoundOfferingMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="imsr_February2025BridgeRoundOfferingMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="us-gaap_DebtInstrumentLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtTableTextBlock" xlink:label="us-gaap_ConvertibleDebtTableTextBlock" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_ConvertibleDebtTableTextBlock" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureOrganizationAndDescriptionOfBusinessDetailss" xlink:type="extended" xlink:title="240101 - Disclosure - Organization and Description of Business (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_OrganizationConsolidationAndPresentationOfFinancialStatementsTable" xlink:label="imsr_OrganizationConsolidationAndPresentationOfFinancialStatementsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesAxis" xlink:label="srt_ConsolidatedEntitiesAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="imsr_OrganizationConsolidationAndPresentationOfFinancialStatementsTable" xlink:to="srt_ConsolidatedEntitiesAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ConsolidatedEntitiesDomain" xlink:label="srt_ConsolidatedEntitiesDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_ConsolidatedEntitiesAxis" xlink:to="srt_ConsolidatedEntitiesDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_TerrestrialEnergyDelawareIncMember" xlink:label="imsr_TerrestrialEnergyDelawareIncMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_ConsolidatedEntitiesDomain" xlink:to="imsr_TerrestrialEnergyDelawareIncMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_TerrestrialEnergyCanadaExchangeIncMember" xlink:label="imsr_TerrestrialEnergyCanadaExchangeIncMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_ConsolidatedEntitiesDomain" xlink:to="imsr_TerrestrialEnergyCanadaExchangeIncMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_OrganizationConsolidationAndPresentationOfFinancialStatementsLineItems" xlink:label="imsr_OrganizationConsolidationAndPresentationOfFinancialStatementsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="imsr_OrganizationConsolidationAndPresentationOfFinancialStatementsLineItems" xlink:to="imsr_OrganizationConsolidationAndPresentationOfFinancialStatementsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="imsr_OrganizationConsolidationAndPresentationOfFinancialStatementsLineItems" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_CommonStockShareExchangeRatio" xlink:label="imsr_CommonStockShareExchangeRatio" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="imsr_OrganizationConsolidationAndPresentationOfFinancialStatementsLineItems" xlink:to="imsr_CommonStockShareExchangeRatio" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="imsr_OrganizationConsolidationAndPresentationOfFinancialStatementsLineItems" xlink:to="us-gaap_PreferredStockParOrStatedValuePerShare" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_PreferredStockShareExchangeRatio" xlink:label="imsr_PreferredStockShareExchangeRatio" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="imsr_OrganizationConsolidationAndPresentationOfFinancialStatementsLineItems" xlink:to="imsr_PreferredStockShareExchangeRatio" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureSignificantAccountingPoliciesVariableInterestEntitiesDetails" xlink:type="extended" xlink:title="240201 - Disclosure - Significant Accounting Policies - Variable Interest Entities (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfVariableInterestEntitiesTable" xlink:label="us-gaap_ScheduleOfVariableInterestEntitiesTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipAxis" xlink:label="srt_OwnershipAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfVariableInterestEntitiesTable" xlink:to="srt_OwnershipAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_OwnershipDomain" xlink:label="srt_OwnershipDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_OwnershipAxis" xlink:to="srt_OwnershipDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_TerrestrialEnergyUsaIncMember" xlink:label="imsr_TerrestrialEnergyUsaIncMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_OwnershipDomain" xlink:to="imsr_TerrestrialEnergyUsaIncMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfVariableInterestEntitiesTable" xlink:to="dei_LegalEntityAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_SwhCapitalLlcMember" xlink:label="imsr_SwhCapitalLlcMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="dei_EntityDomain" xlink:to="imsr_SwhCapitalLlcMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableInterestEntityLineItems" xlink:label="us-gaap_VariableInterestEntityLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_VariableInterestEntityLineItems" xlink:to="us-gaap_ScheduleOfVariableInterestEntitiesTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterestOwnershipPercentageByParent" xlink:label="us-gaap_MinorityInterestOwnershipPercentageByParent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_VariableInterestEntityLineItems" xlink:to="us-gaap_MinorityInterestOwnershipPercentageByParent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners" xlink:label="us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_VariableInterestEntityLineItems" xlink:to="us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureSignificantAccountingPoliciesPropertyAndEquipmentNetDetails" xlink:type="extended" xlink:title="240203 - Disclosure - Significant Accounting Policies - Property and Equipment, Net (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerEquipmentMember" xlink:label="us-gaap_ComputerEquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ComputerEquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SoftwareDevelopmentMember" xlink:label="us-gaap_SoftwareDevelopmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_SoftwareDevelopmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquipmentMember" xlink:label="us-gaap_EquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_EquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="us-gaap_FurnitureAndFixturesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_FurnitureAndFixturesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="srt_RangeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="us-gaap_PropertyPlantAndEquipmentLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:label="us-gaap_PropertyPlantAndEquipmentUsefulLife" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentUsefulLife" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureReverseRecapitalizationDetails" xlink:type="extended" xlink:title="240301 - Disclosure - Reverse Recapitalization (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ReverseRecapitalizationTable" xlink:label="imsr_ReverseRecapitalizationTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="imsr_ReverseRecapitalizationTable" xlink:to="dei_LegalEntityAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_TerrestrialEnergyDevelopmentIncMember" xlink:label="imsr_TerrestrialEnergyDevelopmentIncMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="dei_EntityDomain" xlink:to="imsr_TerrestrialEnergyDevelopmentIncMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="imsr_ReverseRecapitalizationTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrivatePlacementMember" xlink:label="us-gaap_PrivatePlacementMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_PrivatePlacementMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="imsr_ReverseRecapitalizationTable" xlink:to="us-gaap_BusinessAcquisitionAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_HcmIiAcquisitionCorpMember" xlink:label="imsr_HcmIiAcquisitionCorpMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="imsr_HcmIiAcquisitionCorpMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ReverseRecapitalizationLineItems" xlink:label="imsr_ReverseRecapitalizationLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_ReverseRecapitalizationTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="us-gaap_IntangibleAssetsNetExcludingGoodwill" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_IntangibleAssetsNetExcludingGoodwill" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill" xlink:label="us-gaap_Goodwill" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_Goodwill" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockConvertibleConversionRatio" xlink:label="us-gaap_PreferredStockConvertibleConversionRatio" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_PreferredStockConvertibleConversionRatio" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRedeemedOrCalledDuringPeriodShares" xlink:label="us-gaap_StockRedeemedOrCalledDuringPeriodShares" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_StockRedeemedOrCalledDuringPeriodShares" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForRepurchaseOfCommonStock" xlink:label="us-gaap_PaymentsForRepurchaseOfCommonStock" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_PaymentsForRepurchaseOfCommonStock" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockPricePerShare" xlink:label="us-gaap_SaleOfStockPricePerShare" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_SaleOfStockPricePerShare" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ProceedsFromIssuanceOfPrivatePlacementGross" xlink:label="imsr_ProceedsFromIssuanceOfPrivatePlacementGross" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_ProceedsFromIssuanceOfPrivatePlacementGross" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureReverseRecapitalizationReconciliationOfBusinessCombinationToCashFlowsDetails" xlink:type="extended" xlink:title="240302 - Disclosure - Reverse Recapitalization - Reconciliation of Business Combination to Cash Flows (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ReverseRecapitalizationTable" xlink:label="imsr_ReverseRecapitalizationTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="imsr_ReverseRecapitalizationTable" xlink:to="us-gaap_BusinessAcquisitionAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_HcmIiAcquisitionCorpMember" xlink:label="imsr_HcmIiAcquisitionCorpMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="imsr_HcmIiAcquisitionCorpMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="imsr_ReverseRecapitalizationTable" xlink:to="dei_LegalEntityAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_TerrestrialEnergyDevelopmentIncMember" xlink:label="imsr_TerrestrialEnergyDevelopmentIncMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="dei_EntityDomain" xlink:to="imsr_TerrestrialEnergyDevelopmentIncMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ReverseRecapitalizationLineItems" xlink:label="imsr_ReverseRecapitalizationLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_ReverseRecapitalizationTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ProceedsFromTrustAccountNetOfRedemption" xlink:label="imsr_ProceedsFromTrustAccountNetOfRedemption" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_ProceedsFromTrustAccountNetOfRedemption" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement" xlink:label="us-gaap_ProceedsFromIssuanceOfPrivatePlacement" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_ProceedsFromIssuanceOfPrivatePlacement" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForMergerRelatedCosts" xlink:label="us-gaap_PaymentsForMergerRelatedCosts" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_PaymentsForMergerRelatedCosts" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ProceedsFromBusinessCombinationNet" xlink:label="imsr_ProceedsFromBusinessCombinationNet" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_ProceedsFromBusinessCombinationNet" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureReverseRecapitalizationCommonStockIssuedImmediatelyFollowingBusinessCombinationDetails" xlink:type="extended" xlink:title="240303 - Disclosure - Reverse Recapitalization - Common Stock Issued Immediately Following Business Combination (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ReverseRecapitalizationTable" xlink:label="imsr_ReverseRecapitalizationTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="imsr_ReverseRecapitalizationTable" xlink:to="dei_LegalEntityAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_TerrestrialEnergyDevelopmentIncMember" xlink:label="imsr_TerrestrialEnergyDevelopmentIncMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="dei_EntityDomain" xlink:to="imsr_TerrestrialEnergyDevelopmentIncMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ReverseRecapitalizationLineItems" xlink:label="imsr_ReverseRecapitalizationLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_ReverseRecapitalizationTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="us-gaap_PreferredStockSharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_PreferredStockSharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued" xlink:label="us-gaap_CommonStockSharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_CommonStockSharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_TotalOrdinarySharesIssued" xlink:label="imsr_TotalOrdinarySharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_TotalOrdinarySharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_AggregateSharesIssuedRecasted" xlink:label="imsr_AggregateSharesIssuedRecasted" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_AggregateSharesIssuedRecasted" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" xlink:label="us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions" xlink:label="us-gaap_StockIssuedDuringPeriodSharesAcquisitions" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesAcquisitions" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_PreferredExchangeableStockSharesIssued" xlink:label="imsr_PreferredExchangeableStockSharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_PreferredExchangeableStockSharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_CommonExchangeableStockSharesIssued" xlink:label="imsr_CommonExchangeableStockSharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_CommonExchangeableStockSharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_TotalExchangeableStockSharesIssued" xlink:label="imsr_TotalExchangeableStockSharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_TotalExchangeableStockSharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_AggregateExchangeableSharesRecasted" xlink:label="imsr_AggregateExchangeableSharesRecasted" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_AggregateExchangeableSharesRecasted" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ConversionOfPreferredExchangeableSharesToExchangeableShares" xlink:label="imsr_ConversionOfPreferredExchangeableSharesToExchangeableShares" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="13" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_ConversionOfPreferredExchangeableSharesToExchangeableShares" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ConversionOfCommonExchangeableSharesToExchangeableShares" xlink:label="imsr_ConversionOfCommonExchangeableSharesToExchangeableShares" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="14" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_ConversionOfCommonExchangeableSharesToExchangeableShares" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ExchangeableStockSharesIssued" xlink:label="imsr_ExchangeableStockSharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="15" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_ExchangeableStockSharesIssued" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureReverseRecapitalizationTransactionExpensesDetails" xlink:type="extended" xlink:title="240304 - Disclosure - Reverse Recapitalization - Transaction Expenses (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ReverseRecapitalizationTable" xlink:label="imsr_ReverseRecapitalizationTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="imsr_ReverseRecapitalizationTable" xlink:to="dei_LegalEntityAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_TerrestrialEnergyDevelopmentIncMember" xlink:label="imsr_TerrestrialEnergyDevelopmentIncMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="dei_EntityDomain" xlink:to="imsr_TerrestrialEnergyDevelopmentIncMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="imsr_ReverseRecapitalizationTable" xlink:to="us-gaap_BusinessAcquisitionAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_HcmIiAcquisitionCorpMember" xlink:label="imsr_HcmIiAcquisitionCorpMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="imsr_HcmIiAcquisitionCorpMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ReverseRecapitalizationLineItems" xlink:label="imsr_ReverseRecapitalizationLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_ReverseRecapitalizationTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ReverseRecapitalizationExpensesAbstract" xlink:label="imsr_ReverseRecapitalizationExpensesAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="imsr_ReverseRecapitalizationLineItems" xlink:to="imsr_ReverseRecapitalizationExpensesAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfessionalFees" xlink:label="us-gaap_ProfessionalFees" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="imsr_ReverseRecapitalizationExpensesAbstract" xlink:to="us-gaap_ProfessionalFees" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LegalFees" xlink:label="us-gaap_LegalFees" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="imsr_ReverseRecapitalizationExpensesAbstract" xlink:to="us-gaap_LegalFees" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts" xlink:label="us-gaap_BusinessCombinationAcquisitionRelatedCosts" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="imsr_ReverseRecapitalizationExpensesAbstract" xlink:to="us-gaap_BusinessCombinationAcquisitionRelatedCosts" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosurePropertyAndEquipmentNetDetails" xlink:type="extended" xlink:title="240501 - Disclosure - Property and Equipment, Net (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerEquipmentMember" xlink:label="us-gaap_ComputerEquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ComputerEquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ComputerSoftwareMember" xlink:label="imsr_ComputerSoftwareMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="imsr_ComputerSoftwareMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquipmentMember" xlink:label="us-gaap_EquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_EquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="us-gaap_FurnitureAndFixturesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_FurnitureAndFixturesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseholdImprovementsMember" xlink:label="us-gaap_LeaseholdImprovementsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_LeaseholdImprovementsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="us-gaap_PropertyPlantAndEquipmentLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="us-gaap_PropertyPlantAndEquipmentGross" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentGross" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationAndAmortization" xlink:label="us-gaap_DepreciationAndAmortization" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_DepreciationAndAmortization" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureIntangibleAssetsNetDetails" xlink:type="extended" xlink:title="240601 - Disclosure - Intangible Assets, Net (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_IntellectualPropertyRightsMember" xlink:label="imsr_IntellectualPropertyRightsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="imsr_IntellectualPropertyRightsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_PatentsAndTrademarksMember" xlink:label="imsr_PatentsAndTrademarksMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="imsr_PatentsAndTrademarksMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="us-gaap_FiniteLivedIntangibleAssetsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:label="us-gaap_FiniteLivedIntangibleAssetUsefulLife" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetUsefulLife" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="us-gaap_FiniteLivedIntangibleAssetsGross" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsGross" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="us-gaap_FiniteLivedIntangibleAssetsNet" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsNet" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfIntangibleAssets" xlink:label="us-gaap_AmortizationOfIntangibleAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_AmortizationOfIntangibleAssets" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureFinancialInstrumentsDetailss" xlink:type="extended" xlink:title="240701 - Disclosure - Financial Instruments (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable" xlink:label="us-gaap_ScheduleOfCashAndCashEquivalentsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAxis" xlink:label="us-gaap_CashAndCashEquivalentsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfCashAndCashEquivalentsTable" xlink:to="us-gaap_CashAndCashEquivalentsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_CashAndCashEquivalentsAxis" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashMember" xlink:label="us-gaap_CashMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="us-gaap_CashMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashEquivalentsMember" xlink:label="us-gaap_CashEquivalentsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="us-gaap_CashEquivalentsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsMember" xlink:label="us-gaap_MoneyMarketFundsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_CashEquivalentsMember" xlink:to="us-gaap_MoneyMarketFundsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USTreasurySecuritiesMember" xlink:label="us-gaap_USTreasurySecuritiesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_CashEquivalentsMember" xlink:to="us-gaap_USTreasurySecuritiesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfCashAndCashEquivalentsTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="us-gaap_FairValueInputsLevel1Member" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel1Member" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsLineItems" xlink:label="us-gaap_CashAndCashEquivalentsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_CashAndCashEquivalentsLineItems" xlink:to="us-gaap_ScheduleOfCashAndCashEquivalentsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ShortTermInvestmentsAmortizedCost" xlink:label="imsr_ShortTermInvestmentsAmortizedCost" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_CashAndCashEquivalentsLineItems" xlink:to="imsr_ShortTermInvestmentsAmortizedCost" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrealizedGainLossOnInvestments" xlink:label="us-gaap_UnrealizedGainLossOnInvestments" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_CashAndCashEquivalentsLineItems" xlink:to="us-gaap_UnrealizedGainLossOnInvestments" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermInvestments" xlink:label="us-gaap_ShortTermInvestments" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_CashAndCashEquivalentsLineItems" xlink:to="us-gaap_ShortTermInvestments" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_CashAndCashEquivalentsLineItems" xlink:to="us-gaap_CashAndCashEquivalentsAtCarryingValue" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureGovernmentGrantsDetails" xlink:type="extended" xlink:title="241001 - Disclosure - Government Grants (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceTable" xlink:label="us-gaap_GovernmentAssistanceTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_GovernmentAssistanceTable" xlink:to="srt_RangeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceLineItems" xlink:label="us-gaap_GovernmentAssistanceLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_GovernmentAssistanceLineItems" xlink:to="us-gaap_GovernmentAssistanceTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceAwardAmount" xlink:label="us-gaap_GovernmentAssistanceAwardAmount" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_GovernmentAssistanceLineItems" xlink:to="us-gaap_GovernmentAssistanceAwardAmount" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceTransactionDuration" xlink:label="us-gaap_GovernmentAssistanceTransactionDuration" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_GovernmentAssistanceLineItems" xlink:to="us-gaap_GovernmentAssistanceTransactionDuration" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_GovernmentAssistanceRepaymentMultiplier" xlink:label="imsr_GovernmentAssistanceRepaymentMultiplier" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_GovernmentAssistanceLineItems" xlink:to="imsr_GovernmentAssistanceRepaymentMultiplier" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_GovernmentAssistanceAnnualRepaymentRateToRevenue" xlink:label="imsr_GovernmentAssistanceAnnualRepaymentRateToRevenue" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_GovernmentAssistanceLineItems" xlink:to="imsr_GovernmentAssistanceAnnualRepaymentRateToRevenue" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceLiabilityCurrent" xlink:label="us-gaap_GovernmentAssistanceLiabilityCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_GovernmentAssistanceLineItems" xlink:to="us-gaap_GovernmentAssistanceLiabilityCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GovernmentAssistanceLiabilityCurrentStatementOfFinancialPositionExtensibleEnumeration" xlink:label="us-gaap_GovernmentAssistanceLiabilityCurrentStatementOfFinancialPositionExtensibleEnumeration" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_GovernmentAssistanceLiabilityCurrent" xlink:to="us-gaap_GovernmentAssistanceLiabilityCurrentStatementOfFinancialPositionExtensibleEnumeration" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureConvertibleNotesAndConvertibleNotesRelatedParties2024BridgeRoundOfferingDetails" xlink:type="extended" xlink:title="241101 - Disclosure - Convertible Notes and Convertible Notes - Related Parties - 2024 Bridge Round Offering (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_DebtInstrumentAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_BridgeRoundOffering2024Member" xlink:label="imsr_BridgeRoundOffering2024Member" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="imsr_BridgeRoundOffering2024Member" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementScenarioAxis" xlink:label="srt_StatementScenarioAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="srt_StatementScenarioAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="srt_ScenarioUnspecifiedDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_StatementScenarioAxis" xlink:to="srt_ScenarioUnspecifiedDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_AutomaticConversionOfDebtUponSpecialPurposeAcquisitionCompanyMergerMember" xlink:label="imsr_AutomaticConversionOfDebtUponSpecialPurposeAcquisitionCompanyMergerMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_ScenarioUnspecifiedDomain" xlink:to="imsr_AutomaticConversionOfDebtUponSpecialPurposeAcquisitionCompanyMergerMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_AutomaticConversionOfDebtUponAttainingThresholdIssuanceOfCommonAndPreferredSharesExceptSpacSharesMember" xlink:label="imsr_AutomaticConversionOfDebtUponAttainingThresholdIssuanceOfCommonAndPreferredSharesExceptSpacSharesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_ScenarioUnspecifiedDomain" xlink:to="imsr_AutomaticConversionOfDebtUponAttainingThresholdIssuanceOfCommonAndPreferredSharesExceptSpacSharesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_AutomaticConversionOfDebtUponInitialPublicOfferingMember" xlink:label="imsr_AutomaticConversionOfDebtUponInitialPublicOfferingMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="srt_ScenarioUnspecifiedDomain" xlink:to="imsr_AutomaticConversionOfDebtUponInitialPublicOfferingMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeAxis" xlink:label="us-gaap_MeasurementInputTypeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_MeasurementInputTypeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="us-gaap_MeasurementInputTypeDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_MeasurementInputTypeAxis" xlink:to="us-gaap_MeasurementInputTypeDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputSharePriceMember" xlink:label="us-gaap_MeasurementInputSharePriceMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputSharePriceMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputExercisePriceMember" xlink:label="us-gaap_MeasurementInputExercisePriceMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputExercisePriceMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputExpectedTermMember" xlink:label="us-gaap_MeasurementInputExpectedTermMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputExpectedTermMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember" xlink:label="us-gaap_MeasurementInputRiskFreeInterestRateMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputRiskFreeInterestRateMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputPriceVolatilityMember" xlink:label="us-gaap_MeasurementInputPriceVolatilityMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputPriceVolatilityMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="us-gaap_DebtInstrumentLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_NumberOfWarrantsPerUnit" xlink:label="imsr_NumberOfWarrantsPerUnit" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_NumberOfWarrantsPerUnit" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ClassOfWarrantOrRightExercisableExercisePriceOfWarrantsOrRights" xlink:label="imsr_ClassOfWarrantOrRightExercisableExercisePriceOfWarrantsOrRights" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_ClassOfWarrantOrRightExercisableExercisePriceOfWarrantsOrRights" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtInstrumentFaceAmountPerUnit" xlink:label="imsr_DebtInstrumentFaceAmountPerUnit" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtInstrumentFaceAmountPerUnit" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateStatedPercentage" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtInstrumentAutomaticConversionConversionPricePercentageOnIssuePrice" xlink:label="imsr_DebtInstrumentAutomaticConversionConversionPricePercentageOnIssuePrice" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtInstrumentAutomaticConversionConversionPricePercentageOnIssuePrice" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtConversionIssuanceOfAdditionalSecuritiesThresholdPeriodAfterExpiryOfLockUps" xlink:label="imsr_DebtConversionIssuanceOfAdditionalSecuritiesThresholdPeriodAfterExpiryOfLockUps" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtConversionIssuanceOfAdditionalSecuritiesThresholdPeriodAfterExpiryOfLockUps" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtConversionIssuanceOfAdditionalSecuritiesFloorPrice" xlink:label="imsr_DebtConversionIssuanceOfAdditionalSecuritiesFloorPrice" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtConversionIssuanceOfAdditionalSecuritiesFloorPrice" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtInstrumentAutomaticConversionThresholdIssuanceOfCommonOrPreferredShares" xlink:label="imsr_DebtInstrumentAutomaticConversionThresholdIssuanceOfCommonOrPreferredShares" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtInstrumentAutomaticConversionThresholdIssuanceOfCommonOrPreferredShares" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput" xlink:label="us-gaap_WarrantsAndRightsOutstandingMeasurementInput" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_WarrantsAndRightsOutstandingMeasurementInput" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingValuationTechniqueExtensibleList" xlink:label="us-gaap_WarrantsAndRightsOutstandingValuationTechniqueExtensibleList" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_WarrantsAndRightsOutstandingMeasurementInput" xlink:to="us-gaap_WarrantsAndRightsOutstandingValuationTechniqueExtensibleList" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureConvertibleNotesAndConvertibleNotesRelatedPartiesFebruary2025BridgeRoundOfferingDetails" xlink:type="extended" xlink:title="241102 - Disclosure - Convertible Notes and Convertible Notes - Related Parties - February 2025 Bridge Round Offering (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_DebtInstrumentAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_February2025BridgeRoundOfferingMember" xlink:label="imsr_February2025BridgeRoundOfferingMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="imsr_February2025BridgeRoundOfferingMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementScenarioAxis" xlink:label="srt_StatementScenarioAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="srt_StatementScenarioAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="srt_ScenarioUnspecifiedDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_StatementScenarioAxis" xlink:to="srt_ScenarioUnspecifiedDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_AutomaticConversionOfDebtUponSpecialPurposeAcquisitionCompanyMergerMember" xlink:label="imsr_AutomaticConversionOfDebtUponSpecialPurposeAcquisitionCompanyMergerMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_ScenarioUnspecifiedDomain" xlink:to="imsr_AutomaticConversionOfDebtUponSpecialPurposeAcquisitionCompanyMergerMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_AutomaticConversionOfDebtUponAttainingThresholdIssuanceOfCommonAndPreferredSharesExceptSpacSharesMember" xlink:label="imsr_AutomaticConversionOfDebtUponAttainingThresholdIssuanceOfCommonAndPreferredSharesExceptSpacSharesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_ScenarioUnspecifiedDomain" xlink:to="imsr_AutomaticConversionOfDebtUponAttainingThresholdIssuanceOfCommonAndPreferredSharesExceptSpacSharesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_AutomaticConversionOfDebtUponInitialPublicOfferingMember" xlink:label="imsr_AutomaticConversionOfDebtUponInitialPublicOfferingMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="srt_ScenarioUnspecifiedDomain" xlink:to="imsr_AutomaticConversionOfDebtUponInitialPublicOfferingMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeAxis" xlink:label="us-gaap_MeasurementInputTypeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_MeasurementInputTypeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain" xlink:label="us-gaap_MeasurementInputTypeDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_MeasurementInputTypeAxis" xlink:to="us-gaap_MeasurementInputTypeDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputSharePriceMember" xlink:label="us-gaap_MeasurementInputSharePriceMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputSharePriceMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputExercisePriceMember" xlink:label="us-gaap_MeasurementInputExercisePriceMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputExercisePriceMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputExpectedTermMember" xlink:label="us-gaap_MeasurementInputExpectedTermMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputExpectedTermMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember" xlink:label="us-gaap_MeasurementInputRiskFreeInterestRateMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputRiskFreeInterestRateMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputPriceVolatilityMember" xlink:label="us-gaap_MeasurementInputPriceVolatilityMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_MeasurementInputTypeDomain" xlink:to="us-gaap_MeasurementInputPriceVolatilityMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="us-gaap_DebtInstrumentLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromConvertibleDebt" xlink:label="us-gaap_ProceedsFromConvertibleDebt" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_ProceedsFromConvertibleDebt" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_NumberOfWarrantsPerUnit" xlink:label="imsr_NumberOfWarrantsPerUnit" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_NumberOfWarrantsPerUnit" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ClassOfWarrantOrRightExercisableExercisePriceOfWarrantsOrRights" xlink:label="imsr_ClassOfWarrantOrRightExercisableExercisePriceOfWarrantsOrRights" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_ClassOfWarrantOrRightExercisableExercisePriceOfWarrantsOrRights" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtInstrumentFaceAmountPerUnit" xlink:label="imsr_DebtInstrumentFaceAmountPerUnit" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtInstrumentFaceAmountPerUnit" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateStatedPercentage" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1" xlink:label="us-gaap_DebtInstrumentConvertibleConversionPrice1" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentConvertibleConversionPrice1" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtInstrumentAutomaticConversionConversionPricePercentageOnIssuePrice" xlink:label="imsr_DebtInstrumentAutomaticConversionConversionPricePercentageOnIssuePrice" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtInstrumentAutomaticConversionConversionPricePercentageOnIssuePrice" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtConversionIssuanceOfAdditionalSecuritiesThresholdPeriodAfterExpiryOfLockUps" xlink:label="imsr_DebtConversionIssuanceOfAdditionalSecuritiesThresholdPeriodAfterExpiryOfLockUps" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtConversionIssuanceOfAdditionalSecuritiesThresholdPeriodAfterExpiryOfLockUps" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtConversionIssuanceOfAdditionalSecuritiesFloorPrice" xlink:label="imsr_DebtConversionIssuanceOfAdditionalSecuritiesFloorPrice" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtConversionIssuanceOfAdditionalSecuritiesFloorPrice" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtInstrumentAutomaticConversionThresholdIssuanceOfCommonOrPreferredShares" xlink:label="imsr_DebtInstrumentAutomaticConversionThresholdIssuanceOfCommonOrPreferredShares" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtInstrumentAutomaticConversionThresholdIssuanceOfCommonOrPreferredShares" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput" xlink:label="us-gaap_WarrantsAndRightsOutstandingMeasurementInput" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="12" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_WarrantsAndRightsOutstandingMeasurementInput" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingValuationTechniqueExtensibleList" xlink:label="us-gaap_WarrantsAndRightsOutstandingValuationTechniqueExtensibleList" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_WarrantsAndRightsOutstandingMeasurementInput" xlink:to="us-gaap_WarrantsAndRightsOutstandingValuationTechniqueExtensibleList" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureConvertibleNotesAndConvertibleNotesRelatedPartiesRollForwardOfConvertibleNotesAndConvertibleNotesRelatedPartyDetails" xlink:type="extended" xlink:title="241103 - Disclosure - Convertible Notes and Convertible Notes - Related Parties - Roll forward of Convertible Notes and Convertible Notes-Related Party (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable" xlink:label="us-gaap_DebtInstrumentTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_DebtInstrumentAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember" xlink:label="us-gaap_ConvertibleDebtMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="us-gaap_ConvertibleDebtMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonrelatedPartyMember" xlink:label="us-gaap_NonrelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_NonrelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="us-gaap_RelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_RelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="us-gaap_DebtInstrumentLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleLongTermNotesPayable" xlink:label="us-gaap_ConvertibleLongTermNotesPayable" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_ConvertibleLongTermNotesPayable" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromConvertibleDebt" xlink:label="us-gaap_ProceedsFromConvertibleDebt" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_ProceedsFromConvertibleDebt" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:label="us-gaap_GainsLossesOnExtinguishmentOfDebt" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_GainsLossesOnExtinguishmentOfDebt" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtInstrumentDiscount" xlink:label="imsr_DebtInstrumentDiscount" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtInstrumentDiscount" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_DebtInstrumentForeignCurrencyTranslationAdjustment" xlink:label="imsr_DebtInstrumentForeignCurrencyTranslationAdjustment" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="imsr_DebtInstrumentForeignCurrencyTranslationAdjustment" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfDebtDiscountPremium" xlink:label="us-gaap_AmortizationOfDebtDiscountPremium" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_AmortizationOfDebtDiscountPremium" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtConversionOriginalDebtAmount1" xlink:label="us-gaap_DebtConversionOriginalDebtAmount1" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtConversionOriginalDebtAmount1" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureRelatedPartyBalancesAndTransactionsDetailss" xlink:type="extended" xlink:title="241201 - Disclosure - Related Party Balances and Transactions (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember" xlink:label="us-gaap_RelatedPartyMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_RelatedPartyMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_DebtInstrumentAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_February2025BridgeRoundOfferingMember" xlink:label="imsr_February2025BridgeRoundOfferingMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="imsr_February2025BridgeRoundOfferingMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="srt_CounterpartyNameAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="3" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="srt_CounterpartyNameAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_CounterpartyNameAxis" xlink:to="srt_RepurchaseAgreementCounterpartyNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_SwhCapitalLlcMember" xlink:label="imsr_SwhCapitalLlcMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RepurchaseAgreementCounterpartyNameDomain" xlink:to="imsr_SwhCapitalLlcMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="us-gaap_RelatedPartyTransactionLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfessionalFees" xlink:label="us-gaap_ProfessionalFees" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ProfessionalFees" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="us-gaap_ResearchAndDevelopmentExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ResearchAndDevelopmentExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent" xlink:label="us-gaap_AccountsPayableCurrent" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_AccountsPayableCurrent" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions" xlink:label="us-gaap_StockIssuedDuringPeriodSharesAcquisitions" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesAcquisitions" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromRelatedPartyDebt" xlink:label="us-gaap_ProceedsFromRelatedPartyDebt" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ProceedsFromRelatedPartyDebt" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureStockholdersEquityDeficitDetailss" xlink:type="extended" xlink:title="241301 - Disclosure - Stockholders' Equity (Deficit) (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="us-gaap_ScheduleOfStockByClassTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfStockByClassTable" xlink:to="srt_RangeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems" xlink:label="us-gaap_ClassOfStockLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ScheduleOfStockByClassTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:label="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="us-gaap_PreferredStockSharesAuthorized" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_PreferredStockSharesAuthorized" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_PreferredStockParOrStatedValuePerShare" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="us-gaap_PreferredStockSharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_PreferredStockSharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="us-gaap_PreferredStockSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_PreferredStockSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_NumberOfAdditionalSharesIssuedAndConvertedOfPreferredStock" xlink:label="imsr_NumberOfAdditionalSharesIssuedAndConvertedOfPreferredStock" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="imsr_NumberOfAdditionalSharesIssuedAndConvertedOfPreferredStock" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_StockIssuedDuringPeriodSharesConversionOfPreferredStock" xlink:label="imsr_StockIssuedDuringPeriodSharesConversionOfPreferredStock" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_PreferredStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="imsr_StockIssuedDuringPeriodSharesConversionOfPreferredStock" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:label="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="us-gaap_CommonStockSharesAuthorized" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_CommonStockSharesAuthorized" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_CommonStockNumberOfVotesPerShare" xlink:label="imsr_CommonStockNumberOfVotesPerShare" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="imsr_CommonStockNumberOfVotesPerShare" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued" xlink:label="us-gaap_CommonStockSharesIssued" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_CommonStockSharesIssued" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="us-gaap_CommonStockSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_CommonStockNumberOfSharesParValueAndOtherDisclosuresAbstract" xlink:to="us-gaap_CommonStockSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsNoteDisclosureAbstract" xlink:label="us-gaap_WarrantsAndRightsNoteDisclosureAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_WarrantsAndRightsNoteDisclosureAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightOutstanding" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_WarrantsAndRightsNoteDisclosureAbstract" xlink:to="us-gaap_ClassOfWarrantOrRightOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_WarrantsAndRightsNoteDisclosureAbstract" xlink:to="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased" xlink:label="srt_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="srt_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ShareRepurchaseProgramExercisePrice" xlink:label="imsr_ShareRepurchaseProgramExercisePrice" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_ShareRepurchaseProgramExercisePrice" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ExchangeableStockSharesOutstanding" xlink:label="imsr_ExchangeableStockSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_ExchangeableStockSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_CommonExchangeableStockSharesOutstanding" xlink:label="imsr_CommonExchangeableStockSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_CommonExchangeableStockSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_PreferredExchangeableStockSharesOutstanding" xlink:label="imsr_PreferredExchangeableStockSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_PreferredExchangeableStockSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_ExchangeableSharesConversionRatio" xlink:label="imsr_ExchangeableSharesConversionRatio" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_ExchangeableSharesConversionRatio" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_NumberOfSharesExchangedDuringPeriod" xlink:label="imsr_NumberOfSharesExchangedDuringPeriod" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="imsr_NumberOfSharesExchangedDuringPeriod" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureNetLossPerShareOfCommonSharePotentiallyDilutiveSecuritiesExcludedFromCalculationDetailss" xlink:type="extended" xlink:title="241403 - Disclosure - Net Loss Per Share of Common Share - Potentially dilutive securities excluded from the calculation (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:to="us-gaap_AntidilutiveSecuritiesNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_AntidilutiveSecuritiesNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="us-gaap_RestrictedStockUnitsRSUMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AntidilutiveSecuritiesNameDomain" xlink:to="us-gaap_RestrictedStockUnitsRSUMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantMember" xlink:label="us-gaap_WarrantMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_AntidilutiveSecuritiesNameDomain" xlink:to="us-gaap_WarrantMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:to="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureStockBasedCompensationDetailss" xlink:type="extended" xlink:title="241501 - Disclosure - Stock-Based Compensation (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="srt_RangeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureStockBasedCompensationStockBasedCompensationExpenseDetails" xlink:type="extended" xlink:title="241503 - Disclosure - Stock-Based Compensation - Stock-based compensation expense (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureStockBasedCompensationRestrictedStockUnitDetails" xlink:type="extended" xlink:title="241505 - Disclosure - Stock-Based Compensation - Restricted stock unit (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="us-gaap_AwardTypeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="us-gaap_RestrictedStockUnitsRSUMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_RestrictedStockUnitsRSUMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureIncomeTaxesTaxCreditCarryforwardsDetails" xlink:type="extended" xlink:title="241606 - Disclosure - Income Taxes - Tax credit carryforwards (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardTable" xlink:label="us-gaap_TaxCreditCarryforwardTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="us-gaap_IncomeTaxAuthorityAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_TaxCreditCarryforwardTable" xlink:to="us-gaap_IncomeTaxAuthorityAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DomesticCountryMember" xlink:label="us-gaap_DomesticCountryMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_DomesticCountryMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US" xlink:label="country_US" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_DomesticCountryMember" xlink:to="country_US" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StateAndLocalJurisdictionMember" xlink:label="us-gaap_StateAndLocalJurisdictionMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_StateAndLocalJurisdictionMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCountryMember" xlink:label="us-gaap_ForeignCountryMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_ForeignCountryMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CA" xlink:label="country_CA" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_ForeignCountryMember" xlink:to="country_CA" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardLineItems" xlink:label="us-gaap_TaxCreditCarryforwardLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_TaxCreditCarryforwardLineItems" xlink:to="us-gaap_TaxCreditCarryforwardTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwards" xlink:label="us-gaap_OperatingLossCarryforwards" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_TaxCreditCarryforwardLineItems" xlink:to="us-gaap_OperatingLossCarryforwards" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_OperatingLossCarryforwardsExpirationPeriod" xlink:label="imsr_OperatingLossCarryforwardsExpirationPeriod" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_TaxCreditCarryforwardLineItems" xlink:to="imsr_OperatingLossCarryforwardsExpirationPeriod" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardAmount" xlink:label="us-gaap_TaxCreditCarryforwardAmount" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_TaxCreditCarryforwardLineItems" xlink:to="us-gaap_TaxCreditCarryforwardAmount" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_TaxCreditCarryforwardExpirationPeriod" xlink:label="imsr_TaxCreditCarryforwardExpirationPeriod" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_TaxCreditCarryforwardLineItems" xlink:to="imsr_TaxCreditCarryforwardExpirationPeriod" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureSegmentInformationDetailss" xlink:type="extended" xlink:title="241701 - Disclosure - Segment Information (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain" xlink:label="us-gaap_SegmentDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="tmb-20260331.xsd#imsr_SingleReportableSegmentMember" xlink:label="imsr_SingleReportableSegmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="us-gaap_SegmentDomain" xlink:to="imsr_SingleReportableSegmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="us-gaap_SegmentReportingInformationLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfOperatingSegments" xlink:label="us-gaap_NumberOfOperatingSegments" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_NumberOfOperatingSegments" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfReportableSegments" xlink:label="us-gaap_NumberOfReportableSegments" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_NumberOfReportableSegments" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="us-gaap_ResearchAndDevelopmentExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ResearchAndDevelopmentExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpense" xlink:label="us-gaap_GeneralAndAdministrativeExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_GeneralAndAdministrativeExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="us-gaap_DepreciationDepletionAndAmortization" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_DepreciationDepletionAndAmortization" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpenses" xlink:label="us-gaap_OperatingExpenses" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_OperatingExpenses" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="8" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingOtherItemAmount" xlink:label="us-gaap_SegmentReportingOtherItemAmount" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="9" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_SegmentReportingOtherItemAmount" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="us-gaap_IncomeTaxExpenseBenefit" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="10" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_IncomeTaxExpenseBenefit" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="us-gaap_ProfitLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="11" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ProfitLoss" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.terrestrialenergy.com/role/DisclosureSegmentInformationLongLivedAssetsDetailss" xlink:type="extended" xlink:title="241702 - Disclosure - Segment Information - Long-lived assets (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable" xlink:label="us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementAxis" xlink:label="srt_RestatementAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable" xlink:to="srt_RestatementAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RestatementDomain" xlink:label="srt_RestatementDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_RestatementAxis" xlink:to="srt_RestatementDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioPreviouslyReportedMember" xlink:label="srt_ScenarioPreviouslyReportedMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_RestatementDomain" xlink:to="srt_ScenarioPreviouslyReportedMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis" xlink:label="srt_StatementGeographicalAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable" xlink:to="srt_StatementGeographicalAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="srt_SegmentGeographicalDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="srt_StatementGeographicalAxis" xlink:to="srt_SegmentGeographicalDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US" xlink:label="country_US" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="srt_SegmentGeographicalDomain" xlink:to="country_US" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CA" xlink:label="country_CA" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="srt_SegmentGeographicalDomain" xlink:to="country_CA" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems" xlink:label="us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems" xlink:to="us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncurrentAssets" xlink:label="us-gaap_NoncurrentAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems" xlink:to="us-gaap_NoncurrentAssets" priority="1" use="optional" />
  </definitionLink>
</link:linkbase>
