<?xml version="1.0" encoding="US-ASCII" standalone="no"?>
    <!-- Field: Doc-Info; Name: Generator; Value: GoFiler Complete; Version: 6.5c -->
    <!-- Field: Doc-Info; Name: VendorURI; Value: https://www.novaworks.com -->
    <!-- Field: Doc-Info; Name: Status; Value: 0x00000000 -->
<link:linkbase xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xbrli="http://www.xbrl.org/2003/instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#Cover" roleURI="http://fusion-fuel.eu/role/Cover" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ConsolidatedStatementOfFinancialPosition" roleURI="http://fusion-fuel.eu/role/ConsolidatedStatementOfFinancialPosition" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ConsolidatedStatementOfProfitOrLossAndOtherComprehensiveIncome" roleURI="http://fusion-fuel.eu/role/ConsolidatedStatementOfProfitOrLossAndOtherComprehensiveIncome" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ConsolidatedStatementOfChangesInEquity" roleURI="http://fusion-fuel.eu/role/ConsolidatedStatementOfChangesInEquity" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ConsolidatedStatementOfChangesInEquityParenthetical" roleURI="http://fusion-fuel.eu/role/ConsolidatedStatementOfChangesInEquityParenthetical" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ConsolidatedStatementOfCashFlows" roleURI="http://fusion-fuel.eu/role/ConsolidatedStatementOfCashFlows" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#BusinessActivity" roleURI="http://fusion-fuel.eu/role/BusinessActivity" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#BasisOfPreparationAndSignificantAccountingPolicies" roleURI="http://fusion-fuel.eu/role/BasisOfPreparationAndSignificantAccountingPolicies" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#SignificantAccountingJudgementsEstimatesAndAssumptions" roleURI="http://fusion-fuel.eu/role/SignificantAccountingJudgementsEstimatesAndAssumptions" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#RevenueAndSegmentInformation" roleURI="http://fusion-fuel.eu/role/RevenueAndSegmentInformation" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#CostOfSales" roleURI="http://fusion-fuel.eu/role/CostOfSales" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#AdministrationExpenses" roleURI="http://fusion-fuel.eu/role/AdministrationExpenses" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#Share-basedPayments" roleURI="http://fusion-fuel.eu/role/Share-basedPayments" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#Taxation" roleURI="http://fusion-fuel.eu/role/Taxation" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#PropertyPlantAndEquipment" roleURI="http://fusion-fuel.eu/role/PropertyPlantAndEquipment" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#GoodwillAndIntangibleAssets" roleURI="http://fusion-fuel.eu/role/GoodwillAndIntangibleAssets" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#RelatedPartyReceivables" roleURI="http://fusion-fuel.eu/role/RelatedPartyReceivables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#PrepaymentsAndOtherReceivables" roleURI="http://fusion-fuel.eu/role/PrepaymentsAndOtherReceivables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#Inventory" roleURI="http://fusion-fuel.eu/role/Inventory" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#DepositsAdvancesAndPrepayments" roleURI="http://fusion-fuel.eu/role/DepositsAdvancesAndPrepayments" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#OtherCurrentAssets" roleURI="http://fusion-fuel.eu/role/OtherCurrentAssets" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#OtherPayables-Long-term" roleURI="http://fusion-fuel.eu/role/OtherPayables-Long-term" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#TradeAndOtherPayables" roleURI="http://fusion-fuel.eu/role/TradeAndOtherPayables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#Provisions" roleURI="http://fusion-fuel.eu/role/Provisions" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#DerivativeFinancialInstruments" roleURI="http://fusion-fuel.eu/role/DerivativeFinancialInstruments" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#LoansAndBorrowings" roleURI="http://fusion-fuel.eu/role/LoansAndBorrowings" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#FinanceIncomeAndCosts" roleURI="http://fusion-fuel.eu/role/FinanceIncomeAndCosts" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#CostAccruals" roleURI="http://fusion-fuel.eu/role/CostAccruals" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#DeferredIncome" roleURI="http://fusion-fuel.eu/role/DeferredIncome" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ShareCapitalAndSharePremium" roleURI="http://fusion-fuel.eu/role/ShareCapitalAndSharePremium" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#Leases" roleURI="http://fusion-fuel.eu/role/Leases" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#FinancialInstrumentsAndRiskManagement" roleURI="http://fusion-fuel.eu/role/FinancialInstrumentsAndRiskManagement" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#LossPerShare" roleURI="http://fusion-fuel.eu/role/LossPerShare" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#SubsequentEvents" roleURI="http://fusion-fuel.eu/role/SubsequentEvents" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#BasisOfPreparationAndSignificantAccountingPoliciesPolicies" roleURI="http://fusion-fuel.eu/role/BasisOfPreparationAndSignificantAccountingPoliciesPolicies" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#BasisOfPreparationAndSignificantAccountingPoliciesTables" roleURI="http://fusion-fuel.eu/role/BasisOfPreparationAndSignificantAccountingPoliciesTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#RevenueAndSegmentInformationTables" roleURI="http://fusion-fuel.eu/role/RevenueAndSegmentInformationTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#CostOfSalesTables" roleURI="http://fusion-fuel.eu/role/CostOfSalesTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#AdministrationExpensesTables" roleURI="http://fusion-fuel.eu/role/AdministrationExpensesTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#Share-basedPaymentsTables" roleURI="http://fusion-fuel.eu/role/Share-basedPaymentsTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#TaxationTables" roleURI="http://fusion-fuel.eu/role/TaxationTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#PropertyPlantAndEquipmentTables" roleURI="http://fusion-fuel.eu/role/PropertyPlantAndEquipmentTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#GoodwillAndIntangibleAssetsTables" roleURI="http://fusion-fuel.eu/role/GoodwillAndIntangibleAssetsTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#RelatedPartyReceivablesTables" roleURI="http://fusion-fuel.eu/role/RelatedPartyReceivablesTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#PrepaymentsAndOtherReceivablesTables" roleURI="http://fusion-fuel.eu/role/PrepaymentsAndOtherReceivablesTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#InventoryTables" roleURI="http://fusion-fuel.eu/role/InventoryTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#DepositsAdvancesAndPrepaymentsTables" roleURI="http://fusion-fuel.eu/role/DepositsAdvancesAndPrepaymentsTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#OtherCurrentAssetsTables" roleURI="http://fusion-fuel.eu/role/OtherCurrentAssetsTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#OtherPayables-Long-termTables" roleURI="http://fusion-fuel.eu/role/OtherPayables-Long-termTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#TradeAndOtherPayablesTables" roleURI="http://fusion-fuel.eu/role/TradeAndOtherPayablesTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#DerivativeFinancialInstrumentsTables" roleURI="http://fusion-fuel.eu/role/DerivativeFinancialInstrumentsTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#LoansAndBorrowingsTables" roleURI="http://fusion-fuel.eu/role/LoansAndBorrowingsTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#FinanceIncomeAndCostsTables" roleURI="http://fusion-fuel.eu/role/FinanceIncomeAndCostsTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#CostAccrualsTables" roleURI="http://fusion-fuel.eu/role/CostAccrualsTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#DeferredIncomeTables" roleURI="http://fusion-fuel.eu/role/DeferredIncomeTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ShareCapitalAndSharePremiumTables" roleURI="http://fusion-fuel.eu/role/ShareCapitalAndSharePremiumTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#LeasesTables" roleURI="http://fusion-fuel.eu/role/LeasesTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#FinancialInstrumentsAndRiskManagementTables" roleURI="http://fusion-fuel.eu/role/FinancialInstrumentsAndRiskManagementTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#LossPerShareTables" roleURI="http://fusion-fuel.eu/role/LossPerShareTables" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#BusinessActivityDetailsNarrative" roleURI="http://fusion-fuel.eu/role/BusinessActivityDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfDetailedInformationAboutImpactOnEquityDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfDetailedInformationAboutImpactOnEquityDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#BasisOfPreparationAndSignificantAccountingPoliciesDetailsNarrative" roleURI="http://fusion-fuel.eu/role/BasisOfPreparationAndSignificantAccountingPoliciesDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#SignificantAccountingJudgementsEstimatesAndAssumptionsDetailsNarrative" roleURI="http://fusion-fuel.eu/role/SignificantAccountingJudgementsEstimatesAndAssumptionsDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfRevenueDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfRevenueDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfSegmentInformationDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfSegmentInformationDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfCostOfSalesDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfCostOfSalesDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#RevenueAndSegmentInformationDetailsNarrative" roleURI="http://fusion-fuel.eu/role/RevenueAndSegmentInformationDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfAdministrationExpensesDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfAdministrationExpensesDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfReconciliationToStatementOfProfitOrLossDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfReconciliationToStatementOfProfitOrLossDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#AdministrationExpensesDetailsNarrative" roleURI="http://fusion-fuel.eu/role/AdministrationExpensesDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfShareOptionsDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfShareOptionsDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfElementAdvisoryAgreementDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfElementAdvisoryAgreementDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#Share-basedPaymentsDetailsNarrative" roleURI="http://fusion-fuel.eu/role/Share-basedPaymentsDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfIncomeTaxDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfIncomeTaxDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfPropertyPlantAndEquipmentDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfPropertyPlantAndEquipmentDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#TaxationDetailsNarrative" roleURI="http://fusion-fuel.eu/role/TaxationDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfGoodwillAndIntangibleAssetsDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfGoodwillAndIntangibleAssetsDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#PropertyPlantAndEquipmentDetailsNarrative" roleURI="http://fusion-fuel.eu/role/PropertyPlantAndEquipmentDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#GoodwillAndIntangibleAssetsDetailsNarrative" roleURI="http://fusion-fuel.eu/role/GoodwillAndIntangibleAssetsDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfRelatedPartyReceivablesDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfRelatedPartyReceivablesDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfPrepaymentsAndOtherReceivablesDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfPrepaymentsAndOtherReceivablesDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#RelatedPartyReceivablesDetailsNarrative" roleURI="http://fusion-fuel.eu/role/RelatedPartyReceivablesDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfInventoryDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfInventoryDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfDepositsAdvancesAndPrepaymentsCurrentDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfDepositsAdvancesAndPrepaymentsCurrentDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfDepositsAdvancesAndPrepaymentsNonCurrentDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfDepositsAdvancesAndPrepaymentsNonCurrentDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfOtherCurrentAssetsDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfOtherCurrentAssetsDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfOtherPayablesLongTermDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfOtherPayablesLongTermDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#PrepaymentsAndOtherReceivablesDetailsNarrative" roleURI="http://fusion-fuel.eu/role/PrepaymentsAndOtherReceivablesDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfDeferredTaxLiabilityDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfDeferredTaxLiabilityDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfTradeAndOtherPayablesDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfTradeAndOtherPayablesDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#OtherPayables-Long-termDetailsNarrative" roleURI="http://fusion-fuel.eu/role/OtherPayables-Long-termDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#TradeAndOtherPayablesDetailsNarrative" roleURI="http://fusion-fuel.eu/role/TradeAndOtherPayablesDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfDerivativeLiabilitiesDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfDerivativeLiabilitiesDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ProvisionsDetailsNarrative" roleURI="http://fusion-fuel.eu/role/ProvisionsDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfLoansAndBorrowingsDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfLoansAndBorrowingsDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#DerivativeFinancialInstrumentsDetailsNarrative" roleURI="http://fusion-fuel.eu/role/DerivativeFinancialInstrumentsDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfFinanceIncomeAndCostsDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfFinanceIncomeAndCostsDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfNetFinanceCostsDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfNetFinanceCostsDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#LoansAndBorrowingsDetailsNarrative" roleURI="http://fusion-fuel.eu/role/LoansAndBorrowingsDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#FinanceIncomeAndCostsDetailsNarrative" roleURI="http://fusion-fuel.eu/role/FinanceIncomeAndCostsDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfCostAccrualsDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfCostAccrualsDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfDeferredIncomeDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfDeferredIncomeDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfShareCapitalDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfShareCapitalDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfShareCapitalDetailsParenthetical" roleURI="http://fusion-fuel.eu/role/ScheduleOfShareCapitalDetailsParenthetical" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfLeaseLiabilitiesNon-currentAndCurrentDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfLeaseLiabilitiesNon-currentAndCurrentDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfLeaseLiabilitiesDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfLeaseLiabilitiesDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ShareCapitalAndSharePremiumDetailsNarrative" roleURI="http://fusion-fuel.eu/role/ShareCapitalAndSharePremiumDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#LeasesDetailsNarrative" roleURI="http://fusion-fuel.eu/role/LeasesDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfUnobservableInputsInFairValueMeasurementWithinLevel2Details" roleURI="http://fusion-fuel.eu/role/ScheduleOfUnobservableInputsInFairValueMeasurementWithinLevel2Details" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfExpectedContractualMaturitiesDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfExpectedContractualMaturitiesDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfSignificantExchangeRatesDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfSignificantExchangeRatesDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#FinancialInstrumentsAndRiskManagementDetailsNarrative" roleURI="http://fusion-fuel.eu/role/FinancialInstrumentsAndRiskManagementDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfLossPerShareDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfLossPerShareDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#ScheduleOfDilutedWeightedAverageNumberOfOrdinarySharesDetails" roleURI="http://fusion-fuel.eu/role/ScheduleOfDilutedWeightedAverageNumberOfOrdinarySharesDetails" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#LossPerShareDetailsNarrative" roleURI="http://fusion-fuel.eu/role/LossPerShareDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="htoo-20260630.xsd#SubsequentEventsDetailsNarrative" roleURI="http://fusion-fuel.eu/role/SubsequentEventsDetailsNarrative" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" arcroleURI="http://xbrl.org/int/dim/arcrole/all" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#notAll" arcroleURI="http://xbrl.org/int/dim/arcrole/notAll" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/Cover" xlink:title="00000001 - Document - Cover" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ConsolidatedStatementOfFinancialPosition" xlink:title="00000002 - Statement - Condensed consolidated Statement of Financial Position (Unaudited)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ConsolidatedStatementOfProfitOrLossAndOtherComprehensiveIncome" xlink:title="00000003 - Statement - Condensed consolidated Statement of Profit or Loss and Other Comprehensive Income (Unaudited)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ConsolidatedStatementOfChangesInEquity" xlink:title="00000004 - Statement - Condensed consolidated Statement of Changes in Equity (Unaudited)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_StatementOfChangesInEquityTable" xlink:label="loc_ifrs-fullStatementOfChangesInEquityTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullStatementOfChangesInEquityTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComponentsOfEquityAxis" xlink:label="loc_ifrs-fullComponentsOfEquityAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullStatementOfChangesInEquityTable" xlink:to="loc_ifrs-fullComponentsOfEquityAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComponentsOfEquityDomain" xlink:label="loc_ifrs-fullComponentsOfEquityDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullComponentsOfEquityAxis" xlink:to="loc_ifrs-fullComponentsOfEquityDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComponentsOfEquityDomain" xlink:label="loc_ifrs-fullComponentsOfEquityDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullComponentsOfEquityAxis" xlink:to="loc_ifrs-fullComponentsOfEquityDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OrdinarySharesMember" xlink:label="loc_ifrs-fullOrdinarySharesMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullComponentsOfEquityDomain" xlink:to="loc_ifrs-fullOrdinarySharesMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_PreferenceSharesMember" xlink:label="loc_ifrs-fullPreferenceSharesMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullComponentsOfEquityDomain" xlink:to="loc_ifrs-fullPreferenceSharesMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SharePremiumMember" xlink:label="loc_ifrs-fullSharePremiumMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullComponentsOfEquityDomain" xlink:to="loc_ifrs-fullSharePremiumMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NoncontrollingInterestsMember" xlink:label="loc_ifrs-fullNoncontrollingInterestsMember_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullComponentsOfEquityDomain" xlink:to="loc_ifrs-fullNoncontrollingInterestsMember_30" xlink:type="arc" order="31" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ReserveOfSharebasedPaymentsMember" xlink:label="loc_ifrs-fullReserveOfSharebasedPaymentsMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullComponentsOfEquityDomain" xlink:to="loc_ifrs-fullReserveOfSharebasedPaymentsMember_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ReserveOfEquityComponentOfConvertibleInstrumentsMember" xlink:label="loc_ifrs-fullReserveOfEquityComponentOfConvertibleInstrumentsMember_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullComponentsOfEquityDomain" xlink:to="loc_ifrs-fullReserveOfEquityComponentOfConvertibleInstrumentsMember_50" xlink:type="arc" order="51" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ForeignCurrencyTranslationReserveMember" xlink:label="loc_HTOOForeignCurrencyTranslationReserveMember_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullComponentsOfEquityDomain" xlink:to="loc_HTOOForeignCurrencyTranslationReserveMember_60" xlink:type="arc" order="61" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RetainedEarningsMember" xlink:label="loc_ifrs-fullRetainedEarningsMember_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullComponentsOfEquityDomain" xlink:to="loc_ifrs-fullRetainedEarningsMember_70" xlink:type="arc" order="71" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_Equity" xlink:label="loc_ifrs-fullEquity_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullEquity_100" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberOfSharesOutstandingAfterReverseStockSplit" xlink:label="loc_HTOONumberOfSharesOutstandingAfterReverseStockSplit_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberOfSharesOutstandingAfterReverseStockSplit_100" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfSharesOutstanding" xlink:label="loc_ifrs-fullNumberOfSharesOutstanding_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfSharesOutstanding_100" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProfitLoss" xlink:label="loc_ifrs-fullProfitLoss_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullProfitLoss_100" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OtherComprehensiveIncome" xlink:label="loc_ifrs-fullOtherComprehensiveIncome_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullOtherComprehensiveIncome_100" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComprehensiveIncome" xlink:label="loc_ifrs-fullComprehensiveIncome_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullComprehensiveIncome_100" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncreaseDecreaseThroughCommitmentShares" xlink:label="loc_HTOOIncreaseDecreaseThroughCommitmentShares_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIncreaseDecreaseThroughCommitmentShares_100" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberofSharesIssuedForCommitmentShares" xlink:label="loc_HTOONumberofSharesIssuedForCommitmentShares_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberofSharesIssuedForCommitmentShares_100" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseThroughConversionOfConvertibleInstruments" xlink:label="loc_ifrs-fullIncreaseDecreaseThroughConversionOfConvertibleInstruments_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIncreaseDecreaseThroughConversionOfConvertibleInstruments_100" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberofShareIssuedConvertibleNoteinShares" xlink:label="loc_HTOONumberofShareIssuedConvertibleNoteinShares_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberofShareIssuedConvertibleNoteinShares_100" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncreaseDecreaseThroughReverseSplitRounding" xlink:label="loc_HTOOIncreaseDecreaseThroughReverseSplitRounding_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIncreaseDecreaseThroughReverseSplitRounding_100" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberofSharesIssuedForReverseSplitRounding" xlink:label="loc_HTOONumberofSharesIssuedForReverseSplitRounding_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberofSharesIssuedForReverseSplitRounding_100" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncreaseDecreaseThroughKeystoneNoteConversion" xlink:label="loc_HTOOIncreaseDecreaseThroughKeystoneNoteConversion_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIncreaseDecreaseThroughKeystoneNoteConversion_100" xlink:type="arc" order="13" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberofSharesIssuedForKeystoneNoteConversion" xlink:label="loc_HTOONumberofSharesIssuedForKeystoneNoteConversion_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberofSharesIssuedForKeystoneNoteConversion_100" xlink:type="arc" order="14" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseThroughExerciseOfOptions" xlink:label="loc_ifrs-fullIncreaseDecreaseThroughExerciseOfOptions_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIncreaseDecreaseThroughExerciseOfOptions_100" xlink:type="arc" order="15" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberofSharesIssuedForExerciseOfOptions" xlink:label="loc_HTOONumberofSharesIssuedForExerciseOfOptions_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberofSharesIssuedForExerciseOfOptions_100" xlink:type="arc" order="16" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncreaseDecreaseThroughExchangeWarrants" xlink:label="loc_HTOOIncreaseDecreaseThroughExchangeWarrants_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIncreaseDecreaseThroughExchangeWarrants_100" xlink:type="arc" order="17" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncreaseDecreaseThroughTransactionsATMShareSales" xlink:label="loc_HTOOIncreaseDecreaseThroughTransactionsATMShareSales_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIncreaseDecreaseThroughTransactionsATMShareSales_100" xlink:type="arc" order="18" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberofSharesIssuedATMShareSales" xlink:label="loc_HTOONumberofSharesIssuedATMShareSales_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberofSharesIssuedATMShareSales_100" xlink:type="arc" order="19" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncreaseDecreaseThroughExerciseOfOptionsOne" xlink:label="loc_HTOOIncreaseDecreaseThroughExerciseOfOptionsOne_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIncreaseDecreaseThroughExerciseOfOptionsOne_100" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberofSharesIssuedForExerciseOfOptionsOne" xlink:label="loc_HTOONumberofSharesIssuedForExerciseOfOptionsOne_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberofSharesIssuedForExerciseOfOptionsOne_100" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncreaseDecreaseThroughServiceWarrants" xlink:label="loc_HTOOIncreaseDecreaseThroughServiceWarrants_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIncreaseDecreaseThroughServiceWarrants_100" xlink:type="arc" order="22" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberofSharesIssuedForServiceWarrants" xlink:label="loc_HTOONumberofSharesIssuedForServiceWarrants_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberofSharesIssuedForServiceWarrants_100" xlink:type="arc" order="23" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SharebasedPaymentsConversions" xlink:label="loc_HTOOSharebasedPaymentsConversions_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOSharebasedPaymentsConversions_100" xlink:type="arc" order="24" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseThroughSharebasedPaymentTransactions" xlink:label="loc_ifrs-fullIncreaseDecreaseThroughSharebasedPaymentTransactions_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIncreaseDecreaseThroughSharebasedPaymentTransactions_100" xlink:type="arc" order="25" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IssueOfConvertibleNoteReserve" xlink:label="loc_HTOOIssueOfConvertibleNoteReserve_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIssueOfConvertibleNoteReserve_100" xlink:type="arc" order="26" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IssueOfLegacyEquityReserve" xlink:label="loc_HTOOIssueOfLegacyEquityReserve_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIssueOfLegacyEquityReserve_100" xlink:type="arc" order="27" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_PreferenceStockConversionToCommonStock" xlink:label="loc_HTOOPreferenceStockConversionToCommonStock_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOPreferenceStockConversionToCommonStock_100" xlink:type="arc" order="28" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NoncontrollingInterestsContributions" xlink:label="loc_HTOONoncontrollingInterestsContributions_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONoncontrollingInterestsContributions_100" xlink:type="arc" order="29" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ConversionOfWarrants" xlink:label="loc_HTOOConversionOfWarrants_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOConversionOfWarrants_100" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ExerciseOfWarrants" xlink:label="loc_HTOOExerciseOfWarrants_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOExerciseOfWarrants_100" xlink:type="arc" order="31" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ExerciseOfWarrantsInShares" xlink:label="loc_HTOOExerciseOfWarrantsInShares_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOExerciseOfWarrantsInShares_100" xlink:type="arc" order="32" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ElementAdvisoryAgreementsAdvisoryAndMilestoneFees" xlink:label="loc_HTOOElementAdvisoryAgreementsAdvisoryAndMilestoneFees_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOElementAdvisoryAgreementsAdvisoryAndMilestoneFees_100" xlink:type="arc" order="33" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ElementAdvisoryAgreementsAdvisoryAndMilestoneFeesBalanceInShares" xlink:label="loc_HTOOElementAdvisoryAgreementsAdvisoryAndMilestoneFeesBalanceInShares_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOElementAdvisoryAgreementsAdvisoryAndMilestoneFeesBalanceInShares_100" xlink:type="arc" order="34" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_StockIssuedDuringPeriodValueServices" xlink:label="loc_HTOOStockIssuedDuringPeriodValueServices_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOStockIssuedDuringPeriodValueServices_100" xlink:type="arc" order="35" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_StockIssuedDuringPeriodSharesServices" xlink:label="loc_HTOOStockIssuedDuringPeriodSharesServices_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOStockIssuedDuringPeriodSharesServices_100" xlink:type="arc" order="36" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ExerciseOfPrefundedWarrants" xlink:label="loc_HTOOExerciseOfPrefundedWarrants_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOExerciseOfPrefundedWarrants_100" xlink:type="arc" order="37" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ExerciseOfPrefundedWarrantsInShares" xlink:label="loc_HTOOExerciseOfPrefundedWarrantsInShares_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOExerciseOfPrefundedWarrantsInShares_100" xlink:type="arc" order="38" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ExerciseOfServiceWarrants" xlink:label="loc_HTOOExerciseOfServiceWarrants_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOExerciseOfServiceWarrants_100" xlink:type="arc" order="39" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ExerciseOfServiceWarrantsInShares" xlink:label="loc_HTOOExerciseOfServiceWarrantsInShares_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOExerciseOfServiceWarrantsInShares_100" xlink:type="arc" order="40" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ConsolidatedStatementOfChangesInEquityParenthetical" xlink:title="00000005 - Statement - Condensed consolidated Statement of Changes in Equity (Unaudited) (Parenthetical)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ConsolidatedStatementOfCashFlows" xlink:title="00000006 - Statement - Condensed consolidated Statement of Cash Flows (Unaudited)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/BusinessActivity" xlink:title="999007 - Disclosure - Business activity" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/BasisOfPreparationAndSignificantAccountingPolicies" xlink:title="999008 - Disclosure - Basis of preparation and significant accounting policies" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/SignificantAccountingJudgementsEstimatesAndAssumptions" xlink:title="999009 - Disclosure - Significant accounting judgements, estimates and assumptions" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/RevenueAndSegmentInformation" xlink:title="999010 - Disclosure - Revenue and segment information" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/CostOfSales" xlink:title="999011 - Disclosure - Cost of sales" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/AdministrationExpenses" xlink:title="999012 - Disclosure - Administration expenses" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/Share-basedPayments" xlink:title="999013 - Disclosure - Share-based payments" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/Taxation" xlink:title="999014 - Disclosure - Taxation" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/PropertyPlantAndEquipment" xlink:title="999015 - Disclosure - Property, plant and equipment" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/GoodwillAndIntangibleAssets" xlink:title="999016 - Disclosure - Goodwill and intangible assets" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/RelatedPartyReceivables" xlink:title="999017 - Disclosure - Related party receivables" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/PrepaymentsAndOtherReceivables" xlink:title="999018 - Disclosure - Prepayments and other receivables" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/Inventory" xlink:title="999019 - Disclosure - Inventory" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/DepositsAdvancesAndPrepayments" xlink:title="999020 - Disclosure - Deposits, advances and prepayments" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/OtherCurrentAssets" xlink:title="999021 - Disclosure - Other current assets" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/OtherPayables-Long-term" xlink:title="999022 - Disclosure - Other payables - long-term" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/TradeAndOtherPayables" xlink:title="999023 - Disclosure - Trade and other payables" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/Provisions" xlink:title="999024 - Disclosure - Provisions" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/DerivativeFinancialInstruments" xlink:title="999025 - Disclosure - Derivative financial instruments" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/LoansAndBorrowings" xlink:title="999026 - Disclosure - Loans and borrowings" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/FinanceIncomeAndCosts" xlink:title="999027 - Disclosure - Finance income and costs" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/CostAccruals" xlink:title="999028 - Disclosure - Cost accruals" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/DeferredIncome" xlink:title="999029 - Disclosure - Deferred income" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ShareCapitalAndSharePremium" xlink:title="999030 - Disclosure - Share capital and share premium" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/Leases" xlink:title="999031 - Disclosure - Leases" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/FinancialInstrumentsAndRiskManagement" xlink:title="999032 - Disclosure - Financial instruments and risk management" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/LossPerShare" xlink:title="999033 - Disclosure - Loss per share" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/SubsequentEvents" xlink:title="999034 - Disclosure - Subsequent events" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/BasisOfPreparationAndSignificantAccountingPoliciesPolicies" xlink:title="999035 - Disclosure - Basis of preparation and significant accounting policies (Policies)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/BasisOfPreparationAndSignificantAccountingPoliciesTables" xlink:title="999036 - Disclosure - Basis of preparation and significant accounting policies (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/RevenueAndSegmentInformationTables" xlink:title="999037 - Disclosure - Revenue and segment information (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/CostOfSalesTables" xlink:title="999038 - Disclosure - Cost of sales (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/AdministrationExpensesTables" xlink:title="999039 - Disclosure - Administration expenses (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/Share-basedPaymentsTables" xlink:title="999040 - Disclosure - Share-based payments (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/TaxationTables" xlink:title="999041 - Disclosure - Taxation (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/PropertyPlantAndEquipmentTables" xlink:title="999042 - Disclosure - Property, plant and equipment (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/GoodwillAndIntangibleAssetsTables" xlink:title="999043 - Disclosure - Goodwill and intangible assets (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/RelatedPartyReceivablesTables" xlink:title="999044 - Disclosure - Related party receivables (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/PrepaymentsAndOtherReceivablesTables" xlink:title="999045 - Disclosure - Prepayments and other receivables (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/InventoryTables" xlink:title="999046 - Disclosure - Inventory (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/DepositsAdvancesAndPrepaymentsTables" xlink:title="999047 - Disclosure - Deposits, advances and prepayments (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/OtherCurrentAssetsTables" xlink:title="999048 - Disclosure - Other current assets (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/OtherPayables-Long-termTables" xlink:title="999049 - Disclosure - Other payables - long-term (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/TradeAndOtherPayablesTables" xlink:title="999050 - Disclosure - Trade and other payables (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/DerivativeFinancialInstrumentsTables" xlink:title="999051 - Disclosure - Derivative financial instruments (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/LoansAndBorrowingsTables" xlink:title="999052 - Disclosure - Loans and borrowings (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/FinanceIncomeAndCostsTables" xlink:title="999053 - Disclosure - Finance income and costs (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/CostAccrualsTables" xlink:title="999054 - Disclosure - Cost accruals (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/DeferredIncomeTables" xlink:title="999055 - Disclosure - Deferred income (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ShareCapitalAndSharePremiumTables" xlink:title="999056 - Disclosure - Share capital and share premium (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/LeasesTables" xlink:title="999057 - Disclosure - Leases (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/FinancialInstrumentsAndRiskManagementTables" xlink:title="999058 - Disclosure - Financial instruments and risk management (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/LossPerShareTables" xlink:title="999059 - Disclosure - Loss per share (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/BusinessActivityDetailsNarrative" xlink:title="999060 - Disclosure - Business activity (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:label="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SignificantInvestmentsInSubsidiariesAxis" xlink:label="loc_ifrs-fullSignificantInvestmentsInSubsidiariesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullSignificantInvestmentsInSubsidiariesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SignificantInvestmentsInSubsidiariesDomain" xlink:label="loc_ifrs-fullSignificantInvestmentsInSubsidiariesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullSignificantInvestmentsInSubsidiariesAxis" xlink:to="loc_ifrs-fullSignificantInvestmentsInSubsidiariesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SignificantInvestmentsInSubsidiariesDomain" xlink:label="loc_ifrs-fullSignificantInvestmentsInSubsidiariesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullSignificantInvestmentsInSubsidiariesAxis" xlink:to="loc_ifrs-fullSignificantInvestmentsInSubsidiariesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_QualityIndustrialCorpMember" xlink:label="loc_HTOOQualityIndustrialCorpMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullSignificantInvestmentsInSubsidiariesDomain" xlink:to="loc_HTOOQualityIndustrialCorpMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalAxis" xlink:label="loc_ifrs-fullClassesOfShareCapitalAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_DecemberTwoThousandTwentyFiveWarrantMember" xlink:label="loc_HTOODecemberTwoThousandTwentyFiveWarrantMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOODecemberTwoThousandTwentyFiveWarrantMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_PrefundedWarrantsMember" xlink:label="loc_HTOOPrefundedWarrantsMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOPrefundedWarrantsMember_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_PredundedWarrantsMember" xlink:label="loc_HTOOPredundedWarrantsMember_120" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOPredundedWarrantsMember_120" xlink:type="arc" order="121" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ServiceWarrantsMember" xlink:label="loc_HTOOServiceWarrantsMember_130" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOServiceWarrantsMember_130" xlink:type="arc" order="131" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ClassAOrdinarySharesMember" xlink:label="loc_HTOOClassAOrdinarySharesMember_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOClassAOrdinarySharesMember_180" xlink:type="arc" order="182" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SeriesAConvertiblePreferredSharesMember" xlink:label="loc_HTOOSeriesAConvertiblePreferredSharesMember_200" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOSeriesAConvertiblePreferredSharesMember_200" xlink:type="arc" order="202" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfOrdinarySharesAxis" xlink:label="loc_ifrs-fullClassesOfOrdinarySharesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullClassesOfOrdinarySharesAxis" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfOrdinarySharesDomain" xlink:label="loc_ifrs-fullClassesOfOrdinarySharesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfOrdinarySharesAxis" xlink:to="loc_ifrs-fullClassesOfOrdinarySharesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfOrdinarySharesDomain" xlink:label="loc_ifrs-fullClassesOfOrdinarySharesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfOrdinarySharesAxis" xlink:to="loc_ifrs-fullClassesOfOrdinarySharesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ClassAOrdinarySharesMember" xlink:label="loc_HTOOClassAOrdinarySharesMember_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfOrdinarySharesDomain" xlink:to="loc_HTOOClassAOrdinarySharesMember_60" xlink:type="arc" order="61" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_PrefundedWarrantsMember" xlink:label="loc_HTOOPrefundedWarrantsMember_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfOrdinarySharesDomain" xlink:to="loc_HTOOPrefundedWarrantsMember_70" xlink:type="arc" order="71" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_WarrantsMember" xlink:label="loc_ifrs-fullWarrantsMember_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfOrdinarySharesDomain" xlink:to="loc_ifrs-fullWarrantsMember_80" xlink:type="arc" order="81" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_WarrantsTwoMember" xlink:label="loc_HTOOWarrantsTwoMember_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfOrdinarySharesDomain" xlink:to="loc_HTOOWarrantsTwoMember_100" xlink:type="arc" order="101" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfContractsAxis" xlink:label="loc_ifrs-fullTypesOfContractsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullTypesOfContractsAxis" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfContractsDomain" xlink:label="loc_ifrs-fullTypesOfContractsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullTypesOfContractsAxis" xlink:to="loc_ifrs-fullTypesOfContractsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfContractsDomain" xlink:label="loc_ifrs-fullTypesOfContractsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullTypesOfContractsAxis" xlink:to="loc_ifrs-fullTypesOfContractsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_February2026SecuritiesPurchaseAgreementMember" xlink:label="loc_HTOOFebruary2026SecuritiesPurchaseAgreementMember_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfContractsDomain" xlink:to="loc_HTOOFebruary2026SecuritiesPurchaseAgreementMember_60" xlink:type="arc" order="62" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ShareExchangeAgreementMember" xlink:label="loc_HTOOShareExchangeAgreementMember_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfContractsDomain" xlink:to="loc_HTOOShareExchangeAgreementMember_160" xlink:type="arc" order="162" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NonadjustingEventsAfterReportingPeriodAxis" xlink:label="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodAxis" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NonadjustingEventsAfterReportingPeriodDomain" xlink:label="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodAxis" xlink:to="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NonadjustingEventsAfterReportingPeriodDomain" xlink:label="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodAxis" xlink:to="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NonadjustingEventsAfterReportingPeriodMember" xlink:label="loc_HTOONonadjustingEventsAfterReportingPeriodMember_110" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodDomain" xlink:to="loc_HTOONonadjustingEventsAfterReportingPeriodMember_110" xlink:type="arc" order="112" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesAxis" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RoyalUraniumIncMember" xlink:label="loc_HTOORoyalUraniumIncMember_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:to="loc_HTOORoyalUraniumIncMember_150" xlink:type="arc" order="151" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsAxis" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsDomain" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsDomain" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_MineralAndElementAdvisoryAgreementsMember" xlink:label="loc_HTOOMineralAndElementAdvisoryAgreementsMember_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsDomain" xlink:to="loc_HTOOMineralAndElementAdvisoryAgreementsMember_180" xlink:type="arc" order="183" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_OwnershipInterestAcquiredInSubsidiary" xlink:label="loc_HTOOOwnershipInterestAcquiredInSubsidiary_220" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOOwnershipInterestAcquiredInSubsidiary_220" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ReverseShareSplit" xlink:label="loc_HTOOReverseShareSplit_220" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOReverseShareSplit_220" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_StockIssuedDuringPeriodSharesExerciseOfWarrants" xlink:label="loc_HTOOStockIssuedDuringPeriodSharesExerciseOfWarrants_220" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOStockIssuedDuringPeriodSharesExerciseOfWarrants_220" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_WarrantExercisePrice" xlink:label="loc_HTOOWarrantExercisePrice_220" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOWarrantExercisePrice_220" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseInNumberOfOrdinarySharesIssued" xlink:label="loc_ifrs-fullIncreaseDecreaseInNumberOfOrdinarySharesIssued_220" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIncreaseDecreaseInNumberOfOrdinarySharesIssued_220" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ParValuePerShare" xlink:label="loc_ifrs-fullParValuePerShare_220" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullParValuePerShare_220" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProceedsFromIssueOfOrdinaryShares" xlink:label="loc_ifrs-fullProceedsFromIssueOfOrdinaryShares_220" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullProceedsFromIssueOfOrdinaryShares_220" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProportionOfOwnershipInterestInSubsidiary" xlink:label="loc_ifrs-fullProportionOfOwnershipInterestInSubsidiary_220" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullProportionOfOwnershipInterestInSubsidiary_220" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ShareIssueRelatedCost" xlink:label="loc_ifrs-fullShareIssueRelatedCost_220" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullShareIssueRelatedCost_220" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfSharesIssued" xlink:label="loc_ifrs-fullNumberOfSharesIssued_220" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfSharesIssued_220" xlink:type="arc" order="9" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfDetailedInformationAboutImpactOnEquityDetails" xlink:title="999061 - Disclosure - Schedule of detailed information about impact on equity (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementTable" xlink:label="loc_HTOOIfrsStatementTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIfrsStatementTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RestatementTypeAxis" xlink:label="loc_HTOORestatementTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_HTOOIfrsStatementTable" xlink:to="loc_HTOORestatementTypeAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RestatementTypeDomain" xlink:label="loc_HTOORestatementTypeDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_HTOORestatementTypeAxis" xlink:to="loc_HTOORestatementTypeDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RestatementTypeDomain" xlink:label="loc_HTOORestatementTypeDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_HTOORestatementTypeAxis" xlink:to="loc_HTOORestatementTypeDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_PreviouslyStatedMember" xlink:label="loc_ifrs-fullPreviouslyStatedMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOORestatementTypeDomain" xlink:to="loc_ifrs-fullPreviouslyStatedMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RestatedMember" xlink:label="loc_ifrs-fullRestatedMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOORestatementTypeDomain" xlink:to="loc_ifrs-fullRestatedMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_FinancialEffectOfCorrectionsOfAccountingErrorsMember" xlink:label="loc_ifrs-fullFinancialEffectOfCorrectionsOfAccountingErrorsMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOORestatementTypeDomain" xlink:to="loc_ifrs-fullFinancialEffectOfCorrectionsOfAccountingErrorsMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IntangibleAssetsOtherThanGoodwill" xlink:label="loc_ifrs-fullIntangibleAssetsOtherThanGoodwill_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIntangibleAssetsOtherThanGoodwill_40" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_Goodwill" xlink:label="loc_ifrs-fullGoodwill_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullGoodwill_40" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NoncurrentAssets" xlink:label="loc_ifrs-fullNoncurrentAssets_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNoncurrentAssets_40" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_Assets" xlink:label="loc_ifrs-fullAssets_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAssets_40" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OtherNoncurrentPayables" xlink:label="loc_ifrs-fullOtherNoncurrentPayables_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullOtherNoncurrentPayables_40" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NoncurrentLiabilities" xlink:label="loc_ifrs-fullNoncurrentLiabilities_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNoncurrentLiabilities_40" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_Liabilities" xlink:label="loc_ifrs-fullLiabilities_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullLiabilities_40" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NetAssetsLiabilities" xlink:label="loc_ifrs-fullNetAssetsLiabilities_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNetAssetsLiabilities_40" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SharePremium" xlink:label="loc_ifrs-fullSharePremium_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullSharePremium_40" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OtherReserves" xlink:label="loc_ifrs-fullOtherReserves_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullOtherReserves_40" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ForeignCurrencyTranslationReserve" xlink:label="loc_HTOOForeignCurrencyTranslationReserve_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOForeignCurrencyTranslationReserve_40" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RetainedEarnings" xlink:label="loc_ifrs-fullRetainedEarnings_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullRetainedEarnings_40" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NoncontrollingInterests" xlink:label="loc_ifrs-fullNoncontrollingInterests_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNoncontrollingInterests_40" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_Equity" xlink:label="loc_ifrs-fullEquity_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullEquity_40" xlink:type="arc" order="13" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/BasisOfPreparationAndSignificantAccountingPoliciesDetailsNarrative" xlink:title="999062 - Disclosure - Basis of preparation and significant accounting policies (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:label="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RestatementTypeAxis" xlink:label="loc_HTOORestatementTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_HTOORestatementTypeAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RestatementTypeDomain" xlink:label="loc_HTOORestatementTypeDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_HTOORestatementTypeAxis" xlink:to="loc_HTOORestatementTypeDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RestatementTypeDomain" xlink:label="loc_HTOORestatementTypeDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_HTOORestatementTypeAxis" xlink:to="loc_HTOORestatementTypeDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_FinancialEffectOfCorrectionsOfAccountingErrorsMember" xlink:label="loc_ifrs-fullFinancialEffectOfCorrectionsOfAccountingErrorsMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOORestatementTypeDomain" xlink:to="loc_ifrs-fullFinancialEffectOfCorrectionsOfAccountingErrorsMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SignificantInvestmentsInSubsidiariesAxis" xlink:label="loc_ifrs-fullSignificantInvestmentsInSubsidiariesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullSignificantInvestmentsInSubsidiariesAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SignificantInvestmentsInSubsidiariesDomain" xlink:label="loc_ifrs-fullSignificantInvestmentsInSubsidiariesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullSignificantInvestmentsInSubsidiariesAxis" xlink:to="loc_ifrs-fullSignificantInvestmentsInSubsidiariesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SignificantInvestmentsInSubsidiariesDomain" xlink:label="loc_ifrs-fullSignificantInvestmentsInSubsidiariesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullSignificantInvestmentsInSubsidiariesAxis" xlink:to="loc_ifrs-fullSignificantInvestmentsInSubsidiariesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_QualityIndustrialCorpMember" xlink:label="loc_HTOOQualityIndustrialCorpMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullSignificantInvestmentsInSubsidiariesDomain" xlink:to="loc_HTOOQualityIndustrialCorpMember_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesAxis" xlink:label="loc_ifrs-fullCounterpartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullCounterpartiesAxis" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_QualityIndustrialCorpMember" xlink:label="loc_HTOOQualityIndustrialCorpMember_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCounterpartiesDomain" xlink:to="loc_HTOOQualityIndustrialCorpMember_60" xlink:type="arc" order="61" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis" xlink:label="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentDomain" xlink:label="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" xlink:to="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentDomain" xlink:label="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" xlink:to="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MachineryMember" xlink:label="loc_ifrs-fullMachineryMember_110" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" xlink:to="loc_ifrs-fullMachineryMember_110" xlink:type="arc" order="111" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OfficeEquipmentMember" xlink:label="loc_ifrs-fullOfficeEquipmentMember_130" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" xlink:to="loc_ifrs-fullOfficeEquipmentMember_130" xlink:type="arc" order="131" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LeasesMember" xlink:label="loc_HTOOLeasesMember_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" xlink:to="loc_HTOOLeasesMember_150" xlink:type="arc" order="151" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RangeAxis" xlink:label="loc_ifrs-fullRangeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullRangeAxis" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RangeDomain" xlink:label="loc_ifrs-fullRangeDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullRangeAxis" xlink:to="loc_ifrs-fullRangeDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RangeDomain" xlink:label="loc_ifrs-fullRangeDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullRangeAxis" xlink:to="loc_ifrs-fullRangeDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_BottomOfRangeMember" xlink:label="loc_ifrs-fullBottomOfRangeMember_110" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullRangeDomain" xlink:to="loc_ifrs-fullBottomOfRangeMember_110" xlink:type="arc" order="112" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TopOfRangeMember" xlink:label="loc_ifrs-fullTopOfRangeMember_120" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullRangeDomain" xlink:to="loc_ifrs-fullTopOfRangeMember_120" xlink:type="arc" order="122" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:label="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfSignificantInvestmentsInSubsidiariesTable" xlink:to="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillDomain" xlink:label="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:to="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillDomain" xlink:label="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:to="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_BrandNamesMember" xlink:label="loc_ifrs-fullBrandNamesMember_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain" xlink:to="loc_ifrs-fullBrandNamesMember_160" xlink:type="arc" order="161" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ImpairmentLossRecognisedInProfitOrLossGoodwill" xlink:label="loc_ifrs-fullImpairmentLossRecognisedInProfitOrLossGoodwill_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullImpairmentLossRecognisedInProfitOrLossGoodwill_180" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClosingForeignExchangeRate" xlink:label="loc_ifrs-fullClosingForeignExchangeRate_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullClosingForeignExchangeRate_180" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncreaseDecreaseInClosingForeignExchangeRate" xlink:label="loc_HTOOIncreaseDecreaseInClosingForeignExchangeRate_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIncreaseDecreaseInClosingForeignExchangeRate_180" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IntangibleAssetsOfCustomerRelationships" xlink:label="loc_HTOOIntangibleAssetsOfCustomerRelationships_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIntangibleAssetsOfCustomerRelationships_180" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IntangibleAssetsOfTradeNames" xlink:label="loc_HTOOIntangibleAssetsOfTradeNames_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIntangibleAssetsOfTradeNames_180" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseThroughNetExchangeDifferencesGoodwill" xlink:label="loc_ifrs-fullIncreaseDecreaseThroughNetExchangeDifferencesGoodwill_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIncreaseDecreaseThroughNetExchangeDifferencesGoodwill_180" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_InventoryFairValue" xlink:label="loc_HTOOInventoryFairValue_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOInventoryFairValue_180" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseThroughNetExchangeDifferencesDeferredTaxLiabilityAsset" xlink:label="loc_ifrs-fullIncreaseDecreaseThroughNetExchangeDifferencesDeferredTaxLiabilityAsset_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIncreaseDecreaseThroughNetExchangeDifferencesDeferredTaxLiabilityAsset_180" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OtherComprehensiveIncomeBeforeTax" xlink:label="loc_ifrs-fullOtherComprehensiveIncomeBeforeTax_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullOtherComprehensiveIncomeBeforeTax_180" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncreaseDecreaseNoncontrollingInterests" xlink:label="loc_HTOOIncreaseDecreaseNoncontrollingInterests_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIncreaseDecreaseNoncontrollingInterests_180" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ProportionOfOwnershipInterestsHeldByNoncontrollingInterestsShares" xlink:label="loc_HTOOProportionOfOwnershipInterestsHeldByNoncontrollingInterestsShares_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOProportionOfOwnershipInterestsHeldByNoncontrollingInterestsShares_180" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProportionOfVotingRightsHeldByNoncontrollingInterests" xlink:label="loc_ifrs-fullProportionOfVotingRightsHeldByNoncontrollingInterests_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullProportionOfVotingRightsHeldByNoncontrollingInterests_180" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NoncontrollingInterestsShareDescription" xlink:label="loc_HTOONoncontrollingInterestsShareDescription_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONoncontrollingInterestsShareDescription_180" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_OtherComprehensiveIncomeLossDescription" xlink:label="loc_HTOOOtherComprehensiveIncomeLossDescription_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOOtherComprehensiveIncomeLossDescription_180" xlink:type="arc" order="13" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ProfitLossFromOperatingActivity" xlink:label="loc_HTOOProfitLossFromOperatingActivity_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOProfitLossFromOperatingActivity_180" xlink:type="arc" order="14" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProfitLossAttributableToOwnersOfParent" xlink:label="loc_ifrs-fullProfitLossAttributableToOwnersOfParent_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullProfitLossAttributableToOwnersOfParent_180" xlink:type="arc" order="15" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AdjustmentsForSharebasedPayments" xlink:label="loc_ifrs-fullAdjustmentsForSharebasedPayments_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAdjustmentsForSharebasedPayments_180" xlink:type="arc" order="16" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IntangibleAssetsOtherThanGoodwill" xlink:label="loc_ifrs-fullIntangibleAssetsOtherThanGoodwill_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIntangibleAssetsOtherThanGoodwill_180" xlink:type="arc" order="17" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CashFlowsFromUsedInOperatingActivities" xlink:label="loc_ifrs-fullCashFlowsFromUsedInOperatingActivities_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullCashFlowsFromUsedInOperatingActivities_180" xlink:type="arc" order="18" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CurrentAssetsLiabilities" xlink:label="loc_ifrs-fullCurrentAssetsLiabilities_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullCurrentAssetsLiabilities_180" xlink:type="arc" order="19" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CurrentLiabilities" xlink:label="loc_ifrs-fullCurrentLiabilities_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullCurrentLiabilities_180" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CurrentAssets" xlink:label="loc_ifrs-fullCurrentAssets_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullCurrentAssets_180" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_CurrentLiabilitiesIncludePayableToShareholders" xlink:label="loc_HTOOCurrentLiabilitiesIncludePayableToShareholders_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOCurrentLiabilitiesIncludePayableToShareholders_180" xlink:type="arc" order="22" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IssueOfConvertibleInstruments" xlink:label="loc_ifrs-fullIssueOfConvertibleInstruments_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIssueOfConvertibleInstruments_180" xlink:type="arc" order="23" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ShorttermDepositsClassifiedAsCashEquivalents" xlink:label="loc_ifrs-fullShorttermDepositsClassifiedAsCashEquivalents_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullShorttermDepositsClassifiedAsCashEquivalents_180" xlink:type="arc" order="24" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ProceedsFromPrivatePlacement" xlink:label="loc_HTOOProceedsFromPrivatePlacement_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOProceedsFromPrivatePlacement_180" xlink:type="arc" order="25" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_EquityLinesOfCredit" xlink:label="loc_HTOOEquityLinesOfCredit_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOEquityLinesOfCredit_180" xlink:type="arc" order="26" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_UsefulLifeMeasuredAsPeriodOfTimePropertyPlantAndEquipment" xlink:label="loc_ifrs-fullUsefulLifeMeasuredAsPeriodOfTimePropertyPlantAndEquipment_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullUsefulLifeMeasuredAsPeriodOfTimePropertyPlantAndEquipment_180" xlink:type="arc" order="27" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DescriptionOfUsefulLifePropertyPlantAndEquipment" xlink:label="loc_ifrs-fullDescriptionOfUsefulLifePropertyPlantAndEquipment_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDescriptionOfUsefulLifePropertyPlantAndEquipment_180" xlink:type="arc" order="28" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_UsefulLifeMeasuredAsPeriodOfTimeIntangibleAssetsOtherThanGoodwill" xlink:label="loc_ifrs-fullUsefulLifeMeasuredAsPeriodOfTimeIntangibleAssetsOtherThanGoodwill_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullUsefulLifeMeasuredAsPeriodOfTimeIntangibleAssetsOtherThanGoodwill_180" xlink:type="arc" order="29" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/SignificantAccountingJudgementsEstimatesAndAssumptionsDetailsNarrative" xlink:title="999063 - Disclosure - Significant accounting judgements, estimates and assumptions (Details Narrative)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfRevenueDetails" xlink:title="999064 - Disclosure - Schedule of revenue (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:label="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesAxis" xlink:label="loc_ifrs-fullProductsAndServicesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="loc_ifrs-fullProductsAndServicesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesDomain" xlink:label="loc_ifrs-fullProductsAndServicesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullProductsAndServicesAxis" xlink:to="loc_ifrs-fullProductsAndServicesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesDomain" xlink:label="loc_ifrs-fullProductsAndServicesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullProductsAndServicesAxis" xlink:to="loc_ifrs-fullProductsAndServicesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LPGrelatedServicesMember" xlink:label="loc_HTOOLPGrelatedServicesMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullProductsAndServicesDomain" xlink:to="loc_HTOOLPGrelatedServicesMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_EngineeringServicesMember" xlink:label="loc_HTOOEngineeringServicesMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullProductsAndServicesDomain" xlink:to="loc_HTOOEngineeringServicesMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_Revenue" xlink:label="loc_ifrs-fullRevenue_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullRevenue_70" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfSegmentInformationDetails" xlink:title="999065 - Disclosure - Schedule of segment information (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:label="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesAxis" xlink:label="loc_ifrs-fullProductsAndServicesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="loc_ifrs-fullProductsAndServicesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesDomain" xlink:label="loc_ifrs-fullProductsAndServicesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullProductsAndServicesAxis" xlink:to="loc_ifrs-fullProductsAndServicesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesDomain" xlink:label="loc_ifrs-fullProductsAndServicesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullProductsAndServicesAxis" xlink:to="loc_ifrs-fullProductsAndServicesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LPGrelatedServicesMember" xlink:label="loc_HTOOLPGrelatedServicesMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullProductsAndServicesDomain" xlink:to="loc_HTOOLPGrelatedServicesMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_AllOtherActivitiesMember" xlink:label="loc_HTOOAllOtherActivitiesMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullProductsAndServicesDomain" xlink:to="loc_HTOOAllOtherActivitiesMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_Revenue" xlink:label="loc_ifrs-fullRevenue_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullRevenue_40" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_CostOfSale" xlink:label="loc_HTOOCostOfSale_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOCostOfSale_40" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GrossProfit" xlink:label="loc_ifrs-fullGrossProfit_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullGrossProfit_40" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OtherIncome" xlink:label="loc_ifrs-fullOtherIncome_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullOtherIncome_40" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AdministrativeExpense" xlink:label="loc_ifrs-fullAdministrativeExpense_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAdministrativeExpense_40" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AdjustmentsForSharebasedPayments" xlink:label="loc_ifrs-fullAdjustmentsForSharebasedPayments_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAdjustmentsForSharebasedPayments_40" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProfitLossFromOperatingActivities" xlink:label="loc_ifrs-fullProfitLossFromOperatingActivities_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullProfitLossFromOperatingActivities_40" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AdjustmentsForFinanceIncome" xlink:label="loc_ifrs-fullAdjustmentsForFinanceIncome_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAdjustmentsForFinanceIncome_40" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProfitLossBeforeTax" xlink:label="loc_ifrs-fullProfitLossBeforeTax_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullProfitLossBeforeTax_40" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncomeTaxExpenseExcludingTaxExpenseOfAcquiredSubsidiaryAndDeferredTaxLiabilities" xlink:label="loc_HTOOIncomeTaxExpenseExcludingTaxExpenseOfAcquiredSubsidiaryAndDeferredTaxLiabilities_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIncomeTaxExpenseExcludingTaxExpenseOfAcquiredSubsidiaryAndDeferredTaxLiabilities_40" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProfitLoss" xlink:label="loc_ifrs-fullProfitLoss_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullProfitLoss_40" xlink:type="arc" order="10" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfCostOfSalesDetails" xlink:title="999066 - Disclosure - Schedule of cost of sales (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:label="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesAxis" xlink:label="loc_ifrs-fullProductsAndServicesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="loc_ifrs-fullProductsAndServicesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesDomain" xlink:label="loc_ifrs-fullProductsAndServicesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullProductsAndServicesAxis" xlink:to="loc_ifrs-fullProductsAndServicesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesDomain" xlink:label="loc_ifrs-fullProductsAndServicesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullProductsAndServicesAxis" xlink:to="loc_ifrs-fullProductsAndServicesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LPGrelatedServicesQINDAlSholaGasMember" xlink:label="loc_HTOOLPGrelatedServicesQINDAlSholaGasMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullProductsAndServicesDomain" xlink:to="loc_HTOOLPGrelatedServicesQINDAlSholaGasMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_EngineeringServicesBrightHydrogenSolutionsMember" xlink:label="loc_HTOOEngineeringServicesBrightHydrogenSolutionsMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullProductsAndServicesDomain" xlink:to="loc_HTOOEngineeringServicesBrightHydrogenSolutionsMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CostOfSales" xlink:label="loc_ifrs-fullCostOfSales_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullCostOfSales_70" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/RevenueAndSegmentInformationDetailsNarrative" xlink:title="999067 - Disclosure - Revenue and segment information (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:label="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesAxis" xlink:label="loc_ifrs-fullProductsAndServicesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="loc_ifrs-fullProductsAndServicesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesDomain" xlink:label="loc_ifrs-fullProductsAndServicesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullProductsAndServicesAxis" xlink:to="loc_ifrs-fullProductsAndServicesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesDomain" xlink:label="loc_ifrs-fullProductsAndServicesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullProductsAndServicesAxis" xlink:to="loc_ifrs-fullProductsAndServicesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LPGrelatedServicesMember" xlink:label="loc_HTOOLPGrelatedServicesMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullProductsAndServicesDomain" xlink:to="loc_HTOOLPGrelatedServicesMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_Revenue" xlink:label="loc_ifrs-fullRevenue_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullRevenue_50" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AverageForeignExchangeRate" xlink:label="loc_ifrs-fullAverageForeignExchangeRate_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAverageForeignExchangeRate_50" xlink:type="arc" order="1" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfAdministrationExpensesDetails" xlink:title="999068 - Disclosure - Schedule of administration expenses (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfReconciliationToStatementOfProfitOrLossDetails" xlink:title="999069 - Disclosure - Schedule of reconciliation to statement of profit or loss (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementTable" xlink:label="loc_ifrs-fullDisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfSharebasedPaymentArrangementsAxis" xlink:label="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfTermsAndConditionsOfSharebasedPaymentArrangementTable" xlink:to="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfSharebasedPaymentArrangementsDomain" xlink:label="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsAxis" xlink:to="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfSharebasedPaymentArrangementsDomain" xlink:label="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsAxis" xlink:to="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RSUsMember" xlink:label="loc_HTOORSUsMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsDomain" xlink:to="loc_HTOORSUsMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncentiveSharesMember" xlink:label="loc_HTOOIncentiveSharesMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsDomain" xlink:to="loc_HTOOIncentiveSharesMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_OptionsMember" xlink:label="loc_HTOOOptionsMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsDomain" xlink:to="loc_HTOOOptionsMember_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ElementAdvisoryAgreementMember" xlink:label="loc_HTOOElementAdvisoryAgreementMember_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfSharebasedPaymentArrangementsDomain" xlink:to="loc_HTOOElementAdvisoryAgreementMember_60" xlink:type="arc" order="61" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AdjustmentsForSharebasedPayments" xlink:label="loc_ifrs-fullAdjustmentsForSharebasedPayments_110" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAdjustmentsForSharebasedPayments_110" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/AdministrationExpensesDetailsNarrative" xlink:title="999070 - Disclosure - Administration expenses (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfOffsettingOfFinancialAssetsTable" xlink:label="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesAxis" xlink:label="loc_ifrs-fullCounterpartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" xlink:to="loc_ifrs-fullCounterpartiesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_BrightHydrogenSolutionsMember" xlink:label="loc_HTOOBrightHydrogenSolutionsMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCounterpartiesDomain" xlink:to="loc_HTOOBrightHydrogenSolutionsMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_BioSteamEnergyLimitedMember" xlink:label="loc_HTOOBioSteamEnergyLimitedMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCounterpartiesDomain" xlink:to="loc_HTOOBioSteamEnergyLimitedMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_AdministrativeExpenses" xlink:label="loc_HTOOAdministrativeExpenses_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOAdministrativeExpenses_40" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AmortisationIntangibleAssetsOtherThanGoodwill" xlink:label="loc_ifrs-fullAmortisationIntangibleAssetsOtherThanGoodwill_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAmortisationIntangibleAssetsOtherThanGoodwill_40" xlink:type="arc" order="1" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfShareOptionsDetails" xlink:title="999071 - Disclosure - Schedule of share options (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:label="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfAntidilutiveInstrumentsAxis" xlink:label="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:to="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfAntidilutiveInstrumentsDomain" xlink:label="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" xlink:to="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfAntidilutiveInstrumentsDomain" xlink:label="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" xlink:to="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_EmployeesAndDirectorsMember" xlink:label="loc_HTOOEmployeesAndDirectorsMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" xlink:to="loc_HTOOEmployeesAndDirectorsMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ShareBasedCompensationExpense" xlink:label="loc_HTOOShareBasedCompensationExpense_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOShareBasedCompensationExpense_50" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfOutstandingShareOptions" xlink:label="loc_ifrs-fullNumberOfOutstandingShareOptions_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfOutstandingShareOptions_50" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_WeightedAverageExercisePriceOfShareOptionsOutstandingInSharebasedPaymentArrangement2019" xlink:label="loc_ifrs-fullWeightedAverageExercisePriceOfShareOptionsOutstandingInSharebasedPaymentArrangement2019_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullWeightedAverageExercisePriceOfShareOptionsOutstandingInSharebasedPaymentArrangement2019_50" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_WeightedAverageSharePriceShareOptionsOutstanding2019" xlink:label="loc_HTOOWeightedAverageSharePriceShareOptionsOutstanding2019_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOWeightedAverageSharePriceShareOptionsOutstanding2019_50" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfShareOptionsGrantedInSharebasedPaymentArrangement" xlink:label="loc_ifrs-fullNumberOfShareOptionsGrantedInSharebasedPaymentArrangement_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfShareOptionsGrantedInSharebasedPaymentArrangement_50" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfShareOptionsForfeitedInSharebasedPaymentArrangement" xlink:label="loc_ifrs-fullNumberOfShareOptionsForfeitedInSharebasedPaymentArrangement_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfShareOptionsForfeitedInSharebasedPaymentArrangement_50" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfShareOptionsExercisedInSharebasedPaymentArrangement" xlink:label="loc_ifrs-fullNumberOfShareOptionsExercisedInSharebasedPaymentArrangement_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfShareOptionsExercisedInSharebasedPaymentArrangement_50" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfShareOptionsExercisableInSharebasedPaymentArrangement" xlink:label="loc_ifrs-fullNumberOfShareOptionsExercisableInSharebasedPaymentArrangement_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfShareOptionsExercisableInSharebasedPaymentArrangement_50" xlink:type="arc" order="10" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfElementAdvisoryAgreementDetails" xlink:title="999072 - Disclosure - Schedule of element advisory agreement (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_StatementOfChangesInEquityTable" xlink:label="loc_ifrs-fullStatementOfChangesInEquityTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullStatementOfChangesInEquityTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalAxis" xlink:label="loc_ifrs-fullClassesOfShareCapitalAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullStatementOfChangesInEquityTable" xlink:to="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ClassAOrdinarySharesMember" xlink:label="loc_HTOOClassAOrdinarySharesMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOClassAOrdinarySharesMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_PrefundedWarrantsMember" xlink:label="loc_HTOOPrefundedWarrantsMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOPrefundedWarrantsMember_20" xlink:type="arc" order="22" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OrdinarySharesMember" xlink:label="loc_ifrs-fullOrdinarySharesMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_ifrs-fullOrdinarySharesMember_40" xlink:type="arc" order="42" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesAxis" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullStatementOfChangesInEquityTable" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RoyalUraniumIncMember" xlink:label="loc_HTOORoyalUraniumIncMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:to="loc_HTOORoyalUraniumIncMember_0" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsAxis" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullStatementOfChangesInEquityTable" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsDomain" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsDomain" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_MineralAndElementAdvisoryAgreementsMember" xlink:label="loc_HTOOMineralAndElementAdvisoryAgreementsMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsDomain" xlink:to="loc_HTOOMineralAndElementAdvisoryAgreementsMember_0" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfSharesIssued" xlink:label="loc_ifrs-fullNumberOfSharesIssued_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfSharesIssued_90" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ParValuePerShare" xlink:label="loc_ifrs-fullParValuePerShare_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullParValuePerShare_90" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ElementAdvisoryAgreementsAdvisoryAndMilestoneFees" xlink:label="loc_HTOOElementAdvisoryAgreementsAdvisoryAndMilestoneFees_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOElementAdvisoryAgreementsAdvisoryAndMilestoneFees_90" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ElementAdvisoryAgreementsMilestoneFees" xlink:label="loc_HTOOElementAdvisoryAgreementsMilestoneFees_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOElementAdvisoryAgreementsMilestoneFees_90" xlink:type="arc" order="3" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/Share-basedPaymentsDetailsNarrative" xlink:title="999073 - Disclosure - Share-based payments (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:label="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfAntidilutiveInstrumentsAxis" xlink:label="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:to="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfAntidilutiveInstrumentsDomain" xlink:label="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" xlink:to="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfAntidilutiveInstrumentsDomain" xlink:label="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" xlink:to="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RestrictedShareUnitsMember" xlink:label="loc_ifrs-fullRestrictedShareUnitsMember_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" xlink:to="loc_ifrs-fullRestrictedShareUnitsMember_30" xlink:type="arc" order="31" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalAxis" xlink:label="loc_ifrs-fullClassesOfShareCapitalAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:to="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ClassAOrdinarySharesMember" xlink:label="loc_HTOOClassAOrdinarySharesMember_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOClassAOrdinarySharesMember_50" xlink:type="arc" order="51" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ClassAOrdinarySharesOneMember" xlink:label="loc_HTOOClassAOrdinarySharesOneMember_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOClassAOrdinarySharesOneMember_70" xlink:type="arc" order="71" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ClassAOrdinarySharesTwoMember" xlink:label="loc_HTOOClassAOrdinarySharesTwoMember_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOClassAOrdinarySharesTwoMember_80" xlink:type="arc" order="81" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ClassAOrdinarySharesThreeMember" xlink:label="loc_HTOOClassAOrdinarySharesThreeMember_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOClassAOrdinarySharesThreeMember_90" xlink:type="arc" order="91" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_PrefundedWarrantsMember" xlink:label="loc_HTOOPrefundedWarrantsMember_120" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOPrefundedWarrantsMember_120" xlink:type="arc" order="122" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfContractsAxis" xlink:label="loc_ifrs-fullTypesOfContractsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:to="loc_ifrs-fullTypesOfContractsAxis" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfContractsDomain" xlink:label="loc_ifrs-fullTypesOfContractsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullTypesOfContractsAxis" xlink:to="loc_ifrs-fullTypesOfContractsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfContractsDomain" xlink:label="loc_ifrs-fullTypesOfContractsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullTypesOfContractsAxis" xlink:to="loc_ifrs-fullTypesOfContractsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_MineralAndElementAdvisoryAgreementMember" xlink:label="loc_HTOOMineralAndElementAdvisoryAgreementMember_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfContractsDomain" xlink:to="loc_HTOOMineralAndElementAdvisoryAgreementMember_50" xlink:type="arc" order="52" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_TypesOfArrangementAxis" xlink:label="loc_HTOOTypesOfArrangementAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:to="loc_HTOOTypesOfArrangementAxis" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_TypesOfArrangementDomain" xlink:label="loc_HTOOTypesOfArrangementDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_HTOOTypesOfArrangementAxis" xlink:to="loc_HTOOTypesOfArrangementDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_TypesOfArrangementDomain" xlink:label="loc_HTOOTypesOfArrangementDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_HTOOTypesOfArrangementAxis" xlink:to="loc_HTOOTypesOfArrangementDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_MineralAndElementAdvisoryAgreementMember" xlink:label="loc_HTOOMineralAndElementAdvisoryAgreementMember_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOTypesOfArrangementDomain" xlink:to="loc_HTOOMineralAndElementAdvisoryAgreementMember_60" xlink:type="arc" order="62" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesAxis" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RoyalUraniumIncMember" xlink:label="loc_HTOORoyalUraniumIncMember_110" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:to="loc_HTOORoyalUraniumIncMember_110" xlink:type="arc" order="112" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsAxis" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsDomain" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsDomain" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_MineralAndElementAdvisoryAgreementsMember" xlink:label="loc_HTOOMineralAndElementAdvisoryAgreementsMember_110" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsDomain" xlink:to="loc_HTOOMineralAndElementAdvisoryAgreementsMember_110" xlink:type="arc" order="113" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesAxis" xlink:label="loc_ifrs-fullCounterpartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:to="loc_ifrs-fullCounterpartiesAxis" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_BevilacquaPLLCMember" xlink:label="loc_HTOOBevilacquaPLLCMember_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCounterpartiesDomain" xlink:to="loc_HTOOBevilacquaPLLCMember_150" xlink:type="arc" order="151" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_HAndPAdvisoryLimitedMember" xlink:label="loc_HTOOHAndPAdvisoryLimitedMember_170" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCounterpartiesDomain" xlink:to="loc_HTOOHAndPAdvisoryLimitedMember_170" xlink:type="arc" order="171" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ReverseShareSplit" xlink:label="loc_HTOOReverseShareSplit_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOReverseShareSplit_190" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DilutiveEffectOfShareOptionsOnNumberOfOrdinaryShares" xlink:label="loc_ifrs-fullDilutiveEffectOfShareOptionsOnNumberOfOrdinaryShares_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDilutiveEffectOfShareOptionsOnNumberOfOrdinaryShares_190" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ShareBasedCompensationArrangementByShareBasedPaymenEquityInstrumentsOptionsNonvestedNumber" xlink:label="loc_HTOOShareBasedCompensationArrangementByShareBasedPaymenEquityInstrumentsOptionsNonvestedNumber_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOShareBasedCompensationArrangementByShareBasedPaymenEquityInstrumentsOptionsNonvestedNumber_190" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfOtherEquityInstrumentsOutstandingInSharebasedPaymentArrangement" xlink:label="loc_ifrs-fullNumberOfOtherEquityInstrumentsOutstandingInSharebasedPaymentArrangement_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfOtherEquityInstrumentsOutstandingInSharebasedPaymentArrangement_190" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_WeightedAverageExercisePriceOfOtherEquityInstrumentsOutstandingInSharebasedPaymentArrangement2019" xlink:label="loc_ifrs-fullWeightedAverageExercisePriceOfOtherEquityInstrumentsOutstandingInSharebasedPaymentArrangement2019_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullWeightedAverageExercisePriceOfOtherEquityInstrumentsOutstandingInSharebasedPaymentArrangement2019_190" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfSharesIssued" xlink:label="loc_ifrs-fullNumberOfSharesIssued_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfSharesIssued_190" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ParValuePerShare" xlink:label="loc_ifrs-fullParValuePerShare_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullParValuePerShare_190" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_BeneficialOwnershipDescription" xlink:label="loc_HTOOBeneficialOwnershipDescription_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOBeneficialOwnershipDescription_190" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_WarrantsOutstandingTerm" xlink:label="loc_HTOOWarrantsOutstandingTerm_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOWarrantsOutstandingTerm_190" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberOfSharesIssuedValue" xlink:label="loc_HTOONumberOfSharesIssuedValue_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberOfSharesIssuedValue_190" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseThroughSharebasedPaymentTransactions" xlink:label="loc_ifrs-fullIncreaseDecreaseThroughSharebasedPaymentTransactions_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIncreaseDecreaseThroughSharebasedPaymentTransactions_190" xlink:type="arc" order="10" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfIncomeTaxDetails" xlink:title="999074 - Disclosure - Schedule of income tax (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfPropertyPlantAndEquipmentDetails" xlink:title="999075 - Disclosure - Schedule of property plant and equipment (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfReconciliationOfChangesInGoodwillTable" xlink:label="loc_ifrs-fullDisclosureOfReconciliationOfChangesInGoodwillTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfReconciliationOfChangesInGoodwillTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:label="loc_ifrs-fullCarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfReconciliationOfChangesInGoodwillTable" xlink:to="loc_ifrs-fullCarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountMember" xlink:label="loc_ifrs-fullCarryingAmountMember" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:to="loc_ifrs-fullCarryingAmountMember" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountMember" xlink:label="loc_ifrs-fullCarryingAmountMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:to="loc_ifrs-fullCarryingAmountMember_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GrossCarryingAmountMember" xlink:label="loc_ifrs-fullGrossCarryingAmountMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCarryingAmountMember" xlink:to="loc_ifrs-fullGrossCarryingAmountMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AccumulatedDepreciationAndAmortisationMember" xlink:label="loc_ifrs-fullAccumulatedDepreciationAndAmortisationMember_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCarryingAmountMember" xlink:to="loc_ifrs-fullAccumulatedDepreciationAndAmortisationMember_150" xlink:type="arc" order="151" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis" xlink:label="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfReconciliationOfChangesInGoodwillTable" xlink:to="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentDomain" xlink:label="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" xlink:to="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentDomain" xlink:label="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" xlink:to="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_AssetsUnderConstructionMember" xlink:label="loc_HTOOAssetsUnderConstructionMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" xlink:to="loc_HTOOAssetsUnderConstructionMember_0" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MachineryMember" xlink:label="loc_ifrs-fullMachineryMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" xlink:to="loc_ifrs-fullMachineryMember_10" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OfficeEquipmentMember" xlink:label="loc_ifrs-fullOfficeEquipmentMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" xlink:to="loc_ifrs-fullOfficeEquipmentMember_20" xlink:type="arc" order="22" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RightofuseAssetsMember" xlink:label="loc_ifrs-fullRightofuseAssetsMember_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" xlink:to="loc_ifrs-fullRightofuseAssetsMember_30" xlink:type="arc" order="32" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_PropertyPlantAndEquipmentIncludingRightofuseAssets" xlink:label="loc_ifrs-fullPropertyPlantAndEquipmentIncludingRightofuseAssets_410" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullPropertyPlantAndEquipmentIncludingRightofuseAssets_410" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AdditionsOtherThanThroughBusinessCombinationsPropertyPlantAndEquipmentIncludingRightofuseAssets" xlink:label="loc_ifrs-fullAdditionsOtherThanThroughBusinessCombinationsPropertyPlantAndEquipmentIncludingRightofuseAssets_410" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAdditionsOtherThanThroughBusinessCombinationsPropertyPlantAndEquipmentIncludingRightofuseAssets_410" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GainsLossesOnDisposalsOfPropertyPlantAndEquipment" xlink:label="loc_ifrs-fullGainsLossesOnDisposalsOfPropertyPlantAndEquipment_410" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullGainsLossesOnDisposalsOfPropertyPlantAndEquipment_410" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseThroughTransfersPropertyPlantAndEquipmentIncludingRightofuseAssets" xlink:label="loc_ifrs-fullIncreaseDecreaseThroughTransfersPropertyPlantAndEquipmentIncludingRightofuseAssets_410" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIncreaseDecreaseThroughTransfersPropertyPlantAndEquipmentIncludingRightofuseAssets_410" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncreaseDecreaseThroughTransfersPropertyPlantAndEquipmentIncludingRightofuseAssetsExchangeDifferences" xlink:label="loc_HTOOIncreaseDecreaseThroughTransfersPropertyPlantAndEquipmentIncludingRightofuseAssetsExchangeDifferences_410" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIncreaseDecreaseThroughTransfersPropertyPlantAndEquipmentIncludingRightofuseAssetsExchangeDifferences_410" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_DepreciationChargePropertyPlantAndEquipment" xlink:label="loc_HTOODepreciationChargePropertyPlantAndEquipment_410" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOODepreciationChargePropertyPlantAndEquipment_410" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_GainsLossesOnExchangeOfPropertyPlantAndEquipment" xlink:label="loc_HTOOGainsLossesOnExchangeOfPropertyPlantAndEquipment_410" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOGainsLossesOnExchangeOfPropertyPlantAndEquipment_410" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_PropertyPlantAndEquipment" xlink:label="loc_ifrs-fullPropertyPlantAndEquipment_410" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullPropertyPlantAndEquipment_410" xlink:type="arc" order="8" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/TaxationDetailsNarrative" xlink:title="999076 - Disclosure - Taxation (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:label="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GeographicalAreasAxis" xlink:label="loc_ifrs-fullGeographicalAreasAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="loc_ifrs-fullGeographicalAreasAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GeographicalAreasDomain" xlink:label="loc_ifrs-fullGeographicalAreasDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullGeographicalAreasAxis" xlink:to="loc_ifrs-fullGeographicalAreasDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GeographicalAreasDomain" xlink:label="loc_ifrs-fullGeographicalAreasDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullGeographicalAreasAxis" xlink:to="loc_ifrs-fullGeographicalAreasDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_AECountryMember" xlink:label="loc_HTOOAECountryMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullGeographicalAreasDomain" xlink:to="loc_HTOOAECountryMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IrelandCountryMember" xlink:label="loc_HTOOIrelandCountryMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullGeographicalAreasDomain" xlink:to="loc_HTOOIrelandCountryMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SpainCountryMember" xlink:label="loc_HTOOSpainCountryMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullGeographicalAreasDomain" xlink:to="loc_HTOOSpainCountryMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SouthAfricaCountryMember" xlink:label="loc_HTOOSouthAfricaCountryMember_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullGeographicalAreasDomain" xlink:to="loc_HTOOSouthAfricaCountryMember_30" xlink:type="arc" order="31" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ApplicableTaxRate" xlink:label="loc_ifrs-fullApplicableTaxRate_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullApplicableTaxRate_50" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CurrentTaxExpenseIncome" xlink:label="loc_ifrs-fullCurrentTaxExpenseIncome_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullCurrentTaxExpenseIncome_50" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TaxRateEffectOfForeignTaxRates" xlink:label="loc_ifrs-fullTaxRateEffectOfForeignTaxRates_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullTaxRateEffectOfForeignTaxRates_50" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RateOfApplicable" xlink:label="loc_HTOORateOfApplicable_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOORateOfApplicable_50" xlink:type="arc" order="3" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfGoodwillAndIntangibleAssetsDetails" xlink:title="999077 - Disclosure - Schedule of goodwill and intangible assets (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfReconciliationOfChangesInGoodwillTable" xlink:label="loc_ifrs-fullDisclosureOfReconciliationOfChangesInGoodwillTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfReconciliationOfChangesInGoodwillTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:label="loc_ifrs-fullCarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfReconciliationOfChangesInGoodwillTable" xlink:to="loc_ifrs-fullCarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountMember" xlink:label="loc_ifrs-fullCarryingAmountMember" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:to="loc_ifrs-fullCarryingAmountMember" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountMember" xlink:label="loc_ifrs-fullCarryingAmountMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:to="loc_ifrs-fullCarryingAmountMember_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GrossCarryingAmountMember" xlink:label="loc_ifrs-fullGrossCarryingAmountMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCarryingAmountMember" xlink:to="loc_ifrs-fullGrossCarryingAmountMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:label="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfReconciliationOfChangesInGoodwillTable" xlink:to="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillDomain" xlink:label="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:to="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillDomain" xlink:label="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:to="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GoodwillMember" xlink:label="loc_ifrs-fullGoodwillMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain" xlink:to="loc_ifrs-fullGoodwillMember_0" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CustomerrelatedIntangibleAssetsMember" xlink:label="loc_ifrs-fullCustomerrelatedIntangibleAssetsMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain" xlink:to="loc_ifrs-fullCustomerrelatedIntangibleAssetsMember_10" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_BrandNamesMember" xlink:label="loc_ifrs-fullBrandNamesMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfIntangibleAssetsOtherThanGoodwillDomain" xlink:to="loc_ifrs-fullBrandNamesMember_20" xlink:type="arc" order="22" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IntangibleAssetsOtherThanGoodwill" xlink:label="loc_ifrs-fullIntangibleAssetsOtherThanGoodwill_210" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIntangibleAssetsOtherThanGoodwill_210" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AmortisationIntangibleAssetsOtherThanGoodwill" xlink:label="loc_ifrs-fullAmortisationIntangibleAssetsOtherThanGoodwill_210" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAmortisationIntangibleAssetsOtherThanGoodwill_210" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ImpairmentLossRecognisedInProfitOrLossIntangibleAssetsOtherThanGoodwill" xlink:label="loc_ifrs-fullImpairmentLossRecognisedInProfitOrLossIntangibleAssetsOtherThanGoodwill_210" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullImpairmentLossRecognisedInProfitOrLossIntangibleAssetsOtherThanGoodwill_210" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ReversalOfImpairmentLossRecognisedInProfitOrLossIntangibleAssetsOtherThanGoodwill" xlink:label="loc_ifrs-fullReversalOfImpairmentLossRecognisedInProfitOrLossIntangibleAssetsOtherThanGoodwill_210" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullReversalOfImpairmentLossRecognisedInProfitOrLossIntangibleAssetsOtherThanGoodwill_210" xlink:type="arc" order="3" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/PropertyPlantAndEquipmentDetailsNarrative" xlink:title="999078 - Disclosure - Property, plant and equipment (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfPropertyPlantAndEquipmentTable" xlink:label="loc_ifrs-fullDisclosureOfPropertyPlantAndEquipmentTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfPropertyPlantAndEquipmentTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis" xlink:label="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfPropertyPlantAndEquipmentTable" xlink:to="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentDomain" xlink:label="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" xlink:to="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentDomain" xlink:label="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentAxis" xlink:to="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_AOneSholaGasMember" xlink:label="loc_HTOOAOneSholaGasMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfPropertyPlantAndEquipmentDomain" xlink:to="loc_HTOOAOneSholaGasMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AdditionsOtherThanThroughBusinessCombinationsPropertyPlantAndEquipmentIncludingRightofuseAssets" xlink:label="loc_ifrs-fullAdditionsOtherThanThroughBusinessCombinationsPropertyPlantAndEquipmentIncludingRightofuseAssets_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAdditionsOtherThanThroughBusinessCombinationsPropertyPlantAndEquipmentIncludingRightofuseAssets_50" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ConstructionCosts" xlink:label="loc_HTOOConstructionCosts_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOConstructionCosts_50" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DepreciationExpense" xlink:label="loc_ifrs-fullDepreciationExpense_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDepreciationExpense_50" xlink:type="arc" order="2" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/GoodwillAndIntangibleAssetsDetailsNarrative" xlink:title="999079 - Disclosure - Goodwill and intangible assets (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfFairValueMeasurementOfAssetsTable" xlink:label="loc_ifrs-fullDisclosureOfFairValueMeasurementOfAssetsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfFairValueMeasurementOfAssetsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LevelsOfFairValueHierarchyAxis" xlink:label="loc_ifrs-fullLevelsOfFairValueHierarchyAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfFairValueMeasurementOfAssetsTable" xlink:to="loc_ifrs-fullLevelsOfFairValueHierarchyAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LevelsOfFairValueHierarchyDomain" xlink:label="loc_ifrs-fullLevelsOfFairValueHierarchyDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullLevelsOfFairValueHierarchyAxis" xlink:to="loc_ifrs-fullLevelsOfFairValueHierarchyDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LevelsOfFairValueHierarchyDomain" xlink:label="loc_ifrs-fullLevelsOfFairValueHierarchyDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullLevelsOfFairValueHierarchyAxis" xlink:to="loc_ifrs-fullLevelsOfFairValueHierarchyDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_Level3OfFairValueHierarchyMember" xlink:label="loc_ifrs-fullLevel3OfFairValueHierarchyMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullLevelsOfFairValueHierarchyDomain" xlink:to="loc_ifrs-fullLevel3OfFairValueHierarchyMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ImpairmentLossRecognisedInProfitOrLossIntangibleAssetsAndGoodwill" xlink:label="loc_ifrs-fullImpairmentLossRecognisedInProfitOrLossIntangibleAssetsAndGoodwill_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullImpairmentLossRecognisedInProfitOrLossIntangibleAssetsAndGoodwill_50" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ReversalOfImpairmentLoss" xlink:label="loc_ifrs-fullReversalOfImpairmentLoss_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullReversalOfImpairmentLoss_50" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_DisposalCostPercentage" xlink:label="loc_HTOODisposalCostPercentage_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOODisposalCostPercentage_50" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ClosingExchangeRate" xlink:label="loc_HTOOClosingExchangeRate_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOClosingExchangeRate_50" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_GoodwillCarryingAmount" xlink:label="loc_HTOOGoodwillCarryingAmount_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOGoodwillCarryingAmount_50" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NotionalAmount" xlink:label="loc_ifrs-fullNotionalAmount_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNotionalAmount_50" xlink:type="arc" order="5" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfRelatedPartyReceivablesDetails" xlink:title="999080 - Disclosure - Schedule of related party receivables (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:label="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesAxis" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_AOneSholaAOneModeaSafetyAndSecurityLLCMember" xlink:label="loc_HTOOAOneSholaAOneModeaSafetyAndSecurityLLCMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:to="loc_HTOOAOneSholaAOneModeaSafetyAndSecurityLLCMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NoncurrentReceivablesDueFromRelatedParties" xlink:label="loc_ifrs-fullNoncurrentReceivablesDueFromRelatedParties_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNoncurrentReceivablesDueFromRelatedParties_50" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfPrepaymentsAndOtherReceivablesDetails" xlink:title="999081 - Disclosure - Schedule of prepayments and other receivables (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/RelatedPartyReceivablesDetailsNarrative" xlink:title="999082 - Disclosure - Related party receivables (Details Narrative)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfInventoryDetails" xlink:title="999083 - Disclosure - Schedule of inventory (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfFinancialInstrumentsTable" xlink:label="loc_ifrs-fullDisclosureOfFinancialInstrumentsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfFinancialInstrumentsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfFinancialInstrumentsAxis" xlink:label="loc_ifrs-fullClassesOfFinancialInstrumentsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfFinancialInstrumentsTable" xlink:to="loc_ifrs-fullClassesOfFinancialInstrumentsAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfFinancialInstrumentsDomain" xlink:label="loc_ifrs-fullClassesOfFinancialInstrumentsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfFinancialInstrumentsAxis" xlink:to="loc_ifrs-fullClassesOfFinancialInstrumentsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfFinancialInstrumentsDomain" xlink:label="loc_ifrs-fullClassesOfFinancialInstrumentsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfFinancialInstrumentsAxis" xlink:to="loc_ifrs-fullClassesOfFinancialInstrumentsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LPGDistributionMaterialsAndConsumablesQINDAlSholaMember" xlink:label="loc_HTOOLPGDistributionMaterialsAndConsumablesQINDAlSholaMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfFinancialInstrumentsDomain" xlink:to="loc_HTOOLPGDistributionMaterialsAndConsumablesQINDAlSholaMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_InventoriesTotal" xlink:label="loc_ifrs-fullInventoriesTotal_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullInventoriesTotal_50" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_FinishedGoods" xlink:label="loc_ifrs-fullFinishedGoods_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullFinishedGoods_50" xlink:type="arc" order="1" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfDepositsAdvancesAndPrepaymentsCurrentDetails" xlink:title="999084 - Disclosure - Schedule of deposits, advances, and prepayments current (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfFinancialAssetsTable" xlink:label="loc_ifrs-fullDisclosureOfFinancialAssetsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfFinancialAssetsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfFinancialAssetsAxis" xlink:label="loc_ifrs-fullCategoriesOfFinancialAssetsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfFinancialAssetsTable" xlink:to="loc_ifrs-fullCategoriesOfFinancialAssetsAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfFinancialAssetsDomain" xlink:label="loc_ifrs-fullCategoriesOfFinancialAssetsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCategoriesOfFinancialAssetsAxis" xlink:to="loc_ifrs-fullCategoriesOfFinancialAssetsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfFinancialAssetsDomain" xlink:label="loc_ifrs-fullCategoriesOfFinancialAssetsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCategoriesOfFinancialAssetsAxis" xlink:to="loc_ifrs-fullCategoriesOfFinancialAssetsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_DepositsAndAdvancesMember" xlink:label="loc_HTOODepositsAndAdvancesMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfFinancialAssetsDomain" xlink:to="loc_HTOODepositsAndAdvancesMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_OtherPrepaymentsMember" xlink:label="loc_HTOOOtherPrepaymentsMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfFinancialAssetsDomain" xlink:to="loc_HTOOOtherPrepaymentsMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ShorttermDepositsNotClassifiedAsCashEquivalents" xlink:label="loc_ifrs-fullShorttermDepositsNotClassifiedAsCashEquivalents_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullShorttermDepositsNotClassifiedAsCashEquivalents_70" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfDepositsAdvancesAndPrepaymentsNonCurrentDetails" xlink:title="999085 - Disclosure - Schedule of deposits, advances, and prepayments non current (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfOffsettingOfFinancialAssetsTable" xlink:label="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesAxis" xlink:label="loc_ifrs-fullCounterpartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" xlink:to="loc_ifrs-fullCounterpartiesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IndogulfCentralGasLLCMember" xlink:label="loc_HTOOIndogulfCentralGasLLCMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCounterpartiesDomain" xlink:to="loc_HTOOIndogulfCentralGasLLCMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_GeorgiaFireAndRescueSupplyMember" xlink:label="loc_HTOOGeorgiaFireAndRescueSupplyMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCounterpartiesDomain" xlink:to="loc_HTOOGeorgiaFireAndRescueSupplyMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LongtermDeposits" xlink:label="loc_ifrs-fullLongtermDeposits_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullLongtermDeposits_70" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfOtherCurrentAssetsDetails" xlink:title="999086 - Disclosure - Schedule of other current assets (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfOtherPayablesLongTermDetails" xlink:title="999087 - Disclosure - Schedule of other payables long term (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/PrepaymentsAndOtherReceivablesDetailsNarrative" xlink:title="999088 - Disclosure - Prepayments and other receivables (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfOffsettingOfFinancialAssetsTable" xlink:label="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesAxis" xlink:label="loc_ifrs-fullCounterpartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" xlink:to="loc_ifrs-fullCounterpartiesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_AlSholaGasMember" xlink:label="loc_HTOOAlSholaGasMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCounterpartiesDomain" xlink:to="loc_HTOOAlSholaGasMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_BrightHyMember" xlink:label="loc_HTOOBrightHyMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCounterpartiesDomain" xlink:to="loc_HTOOBrightHyMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TradeReceivables" xlink:label="loc_ifrs-fullTradeReceivables_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullTradeReceivables_30" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfDeferredTaxLiabilityDetails" xlink:title="999089 - Disclosure - Schedule of deferred tax liability (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfTradeAndOtherPayablesDetails" xlink:title="999090 - Disclosure - Schedule of trade and other payables (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/OtherPayables-Long-termDetailsNarrative" xlink:title="999091 - Disclosure - Other payables - long-term (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:label="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesAxis" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_AlSholaGasMember" xlink:label="loc_HTOOAlSholaGasMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:to="loc_HTOOAlSholaGasMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_AcquiredInterestPercentage" xlink:label="loc_HTOOAcquiredInterestPercentage_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOAcquiredInterestPercentage_20" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/TradeAndOtherPayablesDetailsNarrative" xlink:title="999092 - Disclosure - Trade and other payables (Details Narrative)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfDerivativeLiabilitiesDetails" xlink:title="999093 - Disclosure - Schedule of derivative liabilities (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfOffsettingOfFinancialLiabilitiesTable" xlink:label="loc_ifrs-fullDisclosureOfOffsettingOfFinancialLiabilitiesTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfOffsettingOfFinancialLiabilitiesTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfFinancialLiabilitiesAxis" xlink:label="loc_ifrs-fullTypesOfFinancialLiabilitiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfOffsettingOfFinancialLiabilitiesTable" xlink:to="loc_ifrs-fullTypesOfFinancialLiabilitiesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfFinancialLiabilitiesDomain" xlink:label="loc_ifrs-fullTypesOfFinancialLiabilitiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullTypesOfFinancialLiabilitiesAxis" xlink:to="loc_ifrs-fullTypesOfFinancialLiabilitiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfFinancialLiabilitiesDomain" xlink:label="loc_ifrs-fullTypesOfFinancialLiabilitiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullTypesOfFinancialLiabilitiesAxis" xlink:to="loc_ifrs-fullTypesOfFinancialLiabilitiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ConvertibleInstrumentsMember" xlink:label="loc_ifrs-fullConvertibleInstrumentsMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfFinancialLiabilitiesDomain" xlink:to="loc_ifrs-fullConvertibleInstrumentsMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_PrivateWarrantsMember" xlink:label="loc_HTOOPrivateWarrantsMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfFinancialLiabilitiesDomain" xlink:to="loc_HTOOPrivateWarrantsMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_PublicWarrantsMember" xlink:label="loc_HTOOPublicWarrantsMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfFinancialLiabilitiesDomain" xlink:to="loc_HTOOPublicWarrantsMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DerivativeFinancialLiabilities" xlink:label="loc_ifrs-fullDerivativeFinancialLiabilities_130" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDerivativeFinancialLiabilities_130" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ForeignExchangeGain" xlink:label="loc_ifrs-fullForeignExchangeGain_130" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullForeignExchangeGain_130" xlink:type="arc" order="1" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ProvisionsDetailsNarrative" xlink:title="999094 - Disclosure - Provisions (Details Narrative)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfLoansAndBorrowingsDetails" xlink:title="999095 - Disclosure - Schedule of loans and borrowings (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/DerivativeFinancialInstrumentsDetailsNarrative" xlink:title="999096 - Disclosure - Derivative financial instruments (Details Narrative)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfFinanceIncomeAndCostsDetails" xlink:title="999097 - Disclosure - Schedule of finance income and costs (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfNetFinanceCostsDetails" xlink:title="999098 - Disclosure - Schedule of net finance costs (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/LoansAndBorrowingsDetailsNarrative" xlink:title="999099 - Disclosure - Loans and borrowings (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfOffsettingOfFinancialAssetsTable" xlink:label="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesAxis" xlink:label="loc_ifrs-fullCounterpartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfOffsettingOfFinancialAssetsTable" xlink:to="loc_ifrs-fullCounterpartiesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_QualityIndustrialCorpMember" xlink:label="loc_HTOOQualityIndustrialCorpMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCounterpartiesDomain" xlink:to="loc_HTOOQualityIndustrialCorpMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IssueOfConvertibleInstruments" xlink:label="loc_ifrs-fullIssueOfConvertibleInstruments_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIssueOfConvertibleInstruments_40" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NotesAndDebenturesIssued" xlink:label="loc_ifrs-fullNotesAndDebenturesIssued_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNotesAndDebenturesIssued_40" xlink:type="arc" order="1" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/FinanceIncomeAndCostsDetailsNarrative" xlink:title="999100 - Disclosure - Finance income and costs (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementTable" xlink:label="loc_HTOOIfrsStatementTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOIfrsStatementTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RestatementTypeAxis" xlink:label="loc_HTOORestatementTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_HTOOIfrsStatementTable" xlink:to="loc_HTOORestatementTypeAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RestatementTypeDomain" xlink:label="loc_HTOORestatementTypeDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_HTOORestatementTypeAxis" xlink:to="loc_HTOORestatementTypeDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RestatementTypeDomain" xlink:label="loc_HTOORestatementTypeDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_HTOORestatementTypeAxis" xlink:to="loc_HTOORestatementTypeDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_PreviouslyStatedMember" xlink:label="loc_ifrs-fullPreviouslyStatedMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOORestatementTypeDomain" xlink:to="loc_ifrs-fullPreviouslyStatedMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OtherComprehensiveIncome" xlink:label="loc_ifrs-fullOtherComprehensiveIncome_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullOtherComprehensiveIncome_20" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComprehensiveIncomeAttributableToOwnersOfParent" xlink:label="loc_ifrs-fullComprehensiveIncomeAttributableToOwnersOfParent_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullComprehensiveIncomeAttributableToOwnersOfParent_20" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComprehensiveIncomeAttributableToNoncontrollingInterests" xlink:label="loc_ifrs-fullComprehensiveIncomeAttributableToNoncontrollingInterests_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullComprehensiveIncomeAttributableToNoncontrollingInterests_20" xlink:type="arc" order="2" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfCostAccrualsDetails" xlink:title="999101 - Disclosure - Schedule of cost accruals (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfDeferredIncomeDetails" xlink:title="999102 - Disclosure - Schedule of deferred Income (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfShareCapitalDetails" xlink:title="999103 - Disclosure - Schedule of share capital (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_StatementOfChangesInEquityTable" xlink:label="loc_ifrs-fullStatementOfChangesInEquityTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullStatementOfChangesInEquityTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalAxis" xlink:label="loc_ifrs-fullClassesOfShareCapitalAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullStatementOfChangesInEquityTable" xlink:to="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ClassAOrdinarySharesMember" xlink:label="loc_HTOOClassAOrdinarySharesMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOClassAOrdinarySharesMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfSharesOutstanding" xlink:label="loc_ifrs-fullNumberOfSharesOutstanding_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfSharesOutstanding_40" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IssuedCapital" xlink:label="loc_ifrs-fullIssuedCapital_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIssuedCapital_40" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SharePremium" xlink:label="loc_ifrs-fullSharePremium_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullSharePremium_40" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberOfOrdinarySharesConversionWarrants" xlink:label="loc_HTOONumberOfOrdinarySharesConversionWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberOfOrdinarySharesConversionWarrants_40" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ShareCapitalConversionWarrants" xlink:label="loc_HTOOShareCapitalConversionWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOShareCapitalConversionWarrants_40" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SharePremiumConversionWarrants" xlink:label="loc_HTOOSharePremiumConversionWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOSharePremiumConversionWarrants_40" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberOfOrdinarySharesExerciseWarrants" xlink:label="loc_HTOONumberOfOrdinarySharesExerciseWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberOfOrdinarySharesExerciseWarrants_40" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ShareCapitalExerciseWarrants" xlink:label="loc_HTOOShareCapitalExerciseWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOShareCapitalExerciseWarrants_40" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SharePremiumExerciseWarrants" xlink:label="loc_HTOOSharePremiumExerciseWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOSharePremiumExerciseWarrants_40" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberOfOrdinarySharesElementAdvisoryAgreements" xlink:label="loc_HTOONumberOfOrdinarySharesElementAdvisoryAgreements_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberOfOrdinarySharesElementAdvisoryAgreements_40" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ShareCapitalElementAdvisoryAgreements" xlink:label="loc_HTOOShareCapitalElementAdvisoryAgreements_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOShareCapitalElementAdvisoryAgreements_40" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SharePremiumElementAdvisoryAgreements" xlink:label="loc_HTOOSharePremiumElementAdvisoryAgreements_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOSharePremiumElementAdvisoryAgreements_40" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberOfOrdinarySharesPrivateInvestmentInPublicEquity" xlink:label="loc_HTOONumberOfOrdinarySharesPrivateInvestmentInPublicEquity_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberOfOrdinarySharesPrivateInvestmentInPublicEquity_40" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ShareCapitalPrivateInvestmentInPublicEquity" xlink:label="loc_HTOOShareCapitalPrivateInvestmentInPublicEquity_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOShareCapitalPrivateInvestmentInPublicEquity_40" xlink:type="arc" order="13" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SharePremiumPrivateInvestmentInPublicEquity" xlink:label="loc_HTOOSharePremiumPrivateInvestmentInPublicEquity_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOSharePremiumPrivateInvestmentInPublicEquity_40" xlink:type="arc" order="14" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberOfOrdinarySharesExerciseOfPrefundedWarrants" xlink:label="loc_HTOONumberOfOrdinarySharesExerciseOfPrefundedWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberOfOrdinarySharesExerciseOfPrefundedWarrants_40" xlink:type="arc" order="15" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ShareCapitalExerciseOfPrefundedWarrants" xlink:label="loc_HTOOShareCapitalExerciseOfPrefundedWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOShareCapitalExerciseOfPrefundedWarrants_40" xlink:type="arc" order="16" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SharePremiumExerciseOfPrefundedWarrants" xlink:label="loc_HTOOSharePremiumExerciseOfPrefundedWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOSharePremiumExerciseOfPrefundedWarrants_40" xlink:type="arc" order="17" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NumberOfOrdinarySharesExerciseOfServiceWarrants" xlink:label="loc_HTOONumberOfOrdinarySharesExerciseOfServiceWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOONumberOfOrdinarySharesExerciseOfServiceWarrants_40" xlink:type="arc" order="18" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ShareCapitalExerciseOfServiceWarrants" xlink:label="loc_HTOOShareCapitalExerciseOfServiceWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOShareCapitalExerciseOfServiceWarrants_40" xlink:type="arc" order="19" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SharePremiumExerciseOfServiceWarrants" xlink:label="loc_HTOOSharePremiumExerciseOfServiceWarrants_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOSharePremiumExerciseOfServiceWarrants_40" xlink:type="arc" order="20" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfShareCapitalDetailsParenthetical" xlink:title="999104 - Disclosure - Schedule of share capital (Details) (Parenthetical)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfLeaseLiabilitiesNon-currentAndCurrentDetails" xlink:title="999105 - Disclosure - Schedule of lease liabilities non-current and current (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfLeaseLiabilitiesDetails" xlink:title="999106 - Disclosure - Schedule of lease liabilities (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ShareCapitalAndSharePremiumDetailsNarrative" xlink:title="999107 - Disclosure - Share capital and share premium (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_StatementOfChangesInEquityTable" xlink:label="loc_ifrs-fullStatementOfChangesInEquityTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullStatementOfChangesInEquityTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalAxis" xlink:label="loc_ifrs-fullClassesOfShareCapitalAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullStatementOfChangesInEquityTable" xlink:to="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ClassAOrdinarySharesMember" xlink:label="loc_HTOOClassAOrdinarySharesMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOClassAOrdinarySharesMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_PreferenceSharesMember" xlink:label="loc_ifrs-fullPreferenceSharesMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_ifrs-fullPreferenceSharesMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfSharesOutstanding" xlink:label="loc_ifrs-fullNumberOfSharesOutstanding_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfSharesOutstanding_60" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ParValuePerShare" xlink:label="loc_ifrs-fullParValuePerShare_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullParValuePerShare_60" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CapitalReserve" xlink:label="loc_ifrs-fullCapitalReserve_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullCapitalReserve_60" xlink:type="arc" order="2" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/LeasesDetailsNarrative" xlink:title="999108 - Disclosure - Leases (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:label="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesAxis" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_QINDAndAlSholaMember" xlink:label="loc_HTOOQINDAndAlSholaMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:to="loc_HTOOQINDAndAlSholaMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_BrightHydrogenSolutionsMember" xlink:label="loc_HTOOBrightHydrogenSolutionsMember_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:to="loc_HTOOBrightHydrogenSolutionsMember_30" xlink:type="arc" order="31" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RangeAxis" xlink:label="loc_ifrs-fullRangeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:to="loc_ifrs-fullRangeAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RangeDomain" xlink:label="loc_ifrs-fullRangeDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullRangeAxis" xlink:to="loc_ifrs-fullRangeDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RangeDomain" xlink:label="loc_ifrs-fullRangeDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullRangeAxis" xlink:to="loc_ifrs-fullRangeDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_BottomOfRangeMember" xlink:label="loc_ifrs-fullBottomOfRangeMember_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullRangeDomain" xlink:to="loc_ifrs-fullBottomOfRangeMember_30" xlink:type="arc" order="32" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TopOfRangeMember" xlink:label="loc_ifrs-fullTopOfRangeMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullRangeDomain" xlink:to="loc_ifrs-fullTopOfRangeMember_40" xlink:type="arc" order="42" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RightofuseAssets" xlink:label="loc_ifrs-fullRightofuseAssets_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullRightofuseAssets_60" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AdjustmentsForInterestExpense" xlink:label="loc_ifrs-fullAdjustmentsForInterestExpense_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAdjustmentsForInterestExpense_60" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_BorrowingsInterestRate" xlink:label="loc_ifrs-fullBorrowingsInterestRate_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullBorrowingsInterestRate_60" xlink:type="arc" order="2" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfUnobservableInputsInFairValueMeasurementWithinLevel2Details" xlink:title="999109 - Disclosure - Schedule of unobservable inputs in fair value measurement within level 2 (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfFairValueMeasurementOfAssetsTable" xlink:label="loc_ifrs-fullDisclosureOfFairValueMeasurementOfAssetsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfFairValueMeasurementOfAssetsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsAxis" xlink:label="loc_ifrs-fullClassesOfAssetsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfFairValueMeasurementOfAssetsTable" xlink:to="loc_ifrs-fullClassesOfAssetsAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsDomain" xlink:label="loc_ifrs-fullClassesOfAssetsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfAssetsAxis" xlink:to="loc_ifrs-fullClassesOfAssetsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsDomain" xlink:label="loc_ifrs-fullClassesOfAssetsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfAssetsAxis" xlink:to="loc_ifrs-fullClassesOfAssetsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_CashAndCashEquivalentMember" xlink:label="loc_HTOOCashAndCashEquivalentMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOCashAndCashEquivalentMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TradeReceivablesMember" xlink:label="loc_ifrs-fullTradeReceivablesMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_ifrs-fullTradeReceivablesMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LoanAndOtherReceivablesMember" xlink:label="loc_HTOOLoanAndOtherReceivablesMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOLoanAndOtherReceivablesMember_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RelatedPartyReceivablesMember" xlink:label="loc_HTOORelatedPartyReceivablesMember_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOORelatedPartyReceivablesMember_60" xlink:type="arc" order="61" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LongtermDepositsMember" xlink:label="loc_HTOOLongtermDepositsMember_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOLongtermDepositsMember_80" xlink:type="arc" order="81" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_TradePayablesMember" xlink:label="loc_HTOOTradePayablesMember_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOTradePayablesMember_100" xlink:type="arc" order="101" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_OtherPayablesMember" xlink:label="loc_HTOOOtherPayablesMember_120" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOOtherPayablesMember_120" xlink:type="arc" order="121" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LoansBankBorrowingsMember" xlink:label="loc_HTOOLoansBankBorrowingsMember_140" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOLoansBankBorrowingsMember_140" xlink:type="arc" order="141" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LeaseLiabilitiesMember" xlink:label="loc_ifrs-fullLeaseLiabilitiesMember_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_ifrs-fullLeaseLiabilitiesMember_160" xlink:type="arc" order="161" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_DerivativeFinancialLiabilitiesMember" xlink:label="loc_HTOODerivativeFinancialLiabilitiesMember_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOODerivativeFinancialLiabilitiesMember_180" xlink:type="arc" order="181" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_OtherEquityReserveMember" xlink:label="loc_HTOOOtherEquityReserveMember_200" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOOtherEquityReserveMember_200" xlink:type="arc" order="201" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LevelsOfFairValueHierarchyAxis" xlink:label="loc_ifrs-fullLevelsOfFairValueHierarchyAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfFairValueMeasurementOfAssetsTable" xlink:to="loc_ifrs-fullLevelsOfFairValueHierarchyAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LevelsOfFairValueHierarchyDomain" xlink:label="loc_ifrs-fullLevelsOfFairValueHierarchyDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullLevelsOfFairValueHierarchyAxis" xlink:to="loc_ifrs-fullLevelsOfFairValueHierarchyDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LevelsOfFairValueHierarchyDomain" xlink:label="loc_ifrs-fullLevelsOfFairValueHierarchyDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullLevelsOfFairValueHierarchyAxis" xlink:to="loc_ifrs-fullLevelsOfFairValueHierarchyDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_Level2OfFairValueHierarchyMember" xlink:label="loc_ifrs-fullLevel2OfFairValueHierarchyMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullLevelsOfFairValueHierarchyDomain" xlink:to="loc_ifrs-fullLevel2OfFairValueHierarchyMember_10" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_CashAndReceivablesCarryingAmount" xlink:label="loc_HTOOCashAndReceivablesCarryingAmount_490" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOCashAndReceivablesCarryingAmount_490" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LiabilitiesAmortisedCost" xlink:label="loc_HTOOLiabilitiesAmortisedCost_490" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOLiabilitiesAmortisedCost_490" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LiabilitiesCarryingAmount" xlink:label="loc_HTOOLiabilitiesCarryingAmount_490" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOLiabilitiesCarryingAmount_490" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_EquityCarryingAmount" xlink:label="loc_HTOOEquityCarryingAmount_490" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOEquityCarryingAmount_490" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_TotalCarryingAmount" xlink:label="loc_HTOOTotalCarryingAmount_490" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOTotalCarryingAmount_490" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_FinancialLiabilitiesAtFairValue" xlink:label="loc_ifrs-fullFinancialLiabilitiesAtFairValue_490" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullFinancialLiabilitiesAtFairValue_490" xlink:type="arc" order="5" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfExpectedContractualMaturitiesDetails" xlink:title="999110 - Disclosure - schedule of expected contractual maturities (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfFairValueMeasurementOfAssetsTable" xlink:label="loc_ifrs-fullDisclosureOfFairValueMeasurementOfAssetsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfFairValueMeasurementOfAssetsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsAxis" xlink:label="loc_ifrs-fullClassesOfAssetsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfFairValueMeasurementOfAssetsTable" xlink:to="loc_ifrs-fullClassesOfAssetsAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsDomain" xlink:label="loc_ifrs-fullClassesOfAssetsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfAssetsAxis" xlink:to="loc_ifrs-fullClassesOfAssetsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsDomain" xlink:label="loc_ifrs-fullClassesOfAssetsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfAssetsAxis" xlink:to="loc_ifrs-fullClassesOfAssetsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_TradePayablesMember" xlink:label="loc_HTOOTradePayablesMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOTradePayablesMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_OtherPayablesMember" xlink:label="loc_HTOOOtherPayablesMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOOtherPayablesMember_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LoansAndBorrowingsCurrentMember" xlink:label="loc_HTOOLoansAndBorrowingsCurrentMember_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOLoansAndBorrowingsCurrentMember_80" xlink:type="arc" order="81" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_BankBorrowingsNonCurrentMember" xlink:label="loc_HTOOBankBorrowingsNonCurrentMember_120" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOBankBorrowingsNonCurrentMember_120" xlink:type="arc" order="121" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_EmployeeEndOfServiceBenefitsMember" xlink:label="loc_HTOOEmployeeEndOfServiceBenefitsMember_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOOEmployeeEndOfServiceBenefitsMember_160" xlink:type="arc" order="161" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LeaseLiabilitiesMember" xlink:label="loc_ifrs-fullLeaseLiabilitiesMember_200" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_ifrs-fullLeaseLiabilitiesMember_200" xlink:type="arc" order="201" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_DerivativeFinancialLiabilitiesMember" xlink:label="loc_HTOODerivativeFinancialLiabilitiesMember_240" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfAssetsDomain" xlink:to="loc_HTOODerivativeFinancialLiabilitiesMember_240" xlink:type="arc" order="241" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MaturityAxis" xlink:label="loc_ifrs-fullMaturityAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfFairValueMeasurementOfAssetsTable" xlink:to="loc_ifrs-fullMaturityAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MaturityDomain" xlink:label="loc_ifrs-fullMaturityDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullMaturityAxis" xlink:to="loc_ifrs-fullMaturityDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MaturityDomain" xlink:label="loc_ifrs-fullMaturityDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullMaturityAxis" xlink:to="loc_ifrs-fullMaturityDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NotLaterThanOneYearMember" xlink:label="loc_ifrs-fullNotLaterThanOneYearMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullMaturityDomain" xlink:to="loc_ifrs-fullNotLaterThanOneYearMember_10" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LaterThanOneYearAndNotLaterThanTwoYearsMember" xlink:label="loc_ifrs-fullLaterThanOneYearAndNotLaterThanTwoYearsMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullMaturityDomain" xlink:to="loc_ifrs-fullLaterThanOneYearAndNotLaterThanTwoYearsMember_20" xlink:type="arc" order="22" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LaterThanTwoYearAndNotLaterThanFiveYearsMember" xlink:label="loc_HTOOLaterThanTwoYearAndNotLaterThanFiveYearsMember_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullMaturityDomain" xlink:to="loc_HTOOLaterThanTwoYearAndNotLaterThanFiveYearsMember_30" xlink:type="arc" order="32" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_FinancialLiabilitiesCarryingAmount" xlink:label="loc_HTOOFinancialLiabilitiesCarryingAmount_330" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOFinancialLiabilitiesCarryingAmount_330" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_FinancialLiabilitiesContractualCashFlows" xlink:label="loc_HTOOFinancialLiabilitiesContractualCashFlows_330" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOFinancialLiabilitiesContractualCashFlows_330" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CurrentDerivativeFinancialLiabilities" xlink:label="loc_ifrs-fullCurrentDerivativeFinancialLiabilities_330" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullCurrentDerivativeFinancialLiabilities_330" xlink:type="arc" order="2" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfSignificantExchangeRatesDetails" xlink:title="999111 - Disclosure - Schedule of significant exchange rates (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:label="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GeographicalAreasAxis" xlink:label="loc_ifrs-fullGeographicalAreasAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="loc_ifrs-fullGeographicalAreasAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GeographicalAreasDomain" xlink:label="loc_ifrs-fullGeographicalAreasDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullGeographicalAreasAxis" xlink:to="loc_ifrs-fullGeographicalAreasDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GeographicalAreasDomain" xlink:label="loc_ifrs-fullGeographicalAreasDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullGeographicalAreasAxis" xlink:to="loc_ifrs-fullGeographicalAreasDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_USDMember" xlink:label="loc_HTOOUSDMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullGeographicalAreasDomain" xlink:to="loc_HTOOUSDMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ZARMember" xlink:label="loc_HTOOZARMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullGeographicalAreasDomain" xlink:to="loc_HTOOZARMember_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AverageForeignExchangeRate" xlink:label="loc_ifrs-fullAverageForeignExchangeRate_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullAverageForeignExchangeRate_80" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClosingForeignExchangeRate" xlink:label="loc_ifrs-fullClosingForeignExchangeRate_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullClosingForeignExchangeRate_80" xlink:type="arc" order="1" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/FinancialInstrumentsAndRiskManagementDetailsNarrative" xlink:title="999112 - Disclosure - Financial instruments and risk management (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:label="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesAxis" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_QINDAndAlSholaMember" xlink:label="loc_HTOOQINDAndAlSholaMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:to="loc_HTOOQINDAndAlSholaMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CashAndCashEquivalents" xlink:label="loc_ifrs-fullCashAndCashEquivalents_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullCashAndCashEquivalents_40" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DerivativeFinancialLiabilities" xlink:label="loc_ifrs-fullDerivativeFinancialLiabilities_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDerivativeFinancialLiabilities_40" xlink:type="arc" order="1" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfLossPerShareDetails" xlink:title="999113 - Disclosure - Schedule of loss per share (Details)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/ScheduleOfDilutedWeightedAverageNumberOfOrdinarySharesDetails" xlink:title="999114 - Disclosure - Schedule of diluted weighted average number of ordinary shares (Details)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:label="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfAntidilutiveInstrumentsAxis" xlink:label="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfInstrumentsWithPotentialFutureDilutiveEffectNotIncludedInCalculationOfDilutedEarningsPerShareTable" xlink:to="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfAntidilutiveInstrumentsDomain" xlink:label="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" xlink:to="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfAntidilutiveInstrumentsDomain" xlink:label="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsAxis" xlink:to="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_WarrantsMember" xlink:label="loc_ifrs-fullWarrantsMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" xlink:to="loc_ifrs-fullWarrantsMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RsusVestedMember" xlink:label="loc_HTOORsusVestedMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" xlink:to="loc_HTOORsusVestedMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IncentiveSharesMember" xlink:label="loc_HTOOIncentiveSharesMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" xlink:to="loc_HTOOIncentiveSharesMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ShareOptionsMember" xlink:label="loc_ifrs-fullShareOptionsMember_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullTypesOfAntidilutiveInstrumentsDomain" xlink:to="loc_ifrs-fullShareOptionsMember_30" xlink:type="arc" order="31" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfPotentialOrdinarySharesThatAreAntidilutiveInPeriodPresented" xlink:label="loc_ifrs-fullNumberOfPotentialOrdinarySharesThatAreAntidilutiveInPeriodPresented_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfPotentialOrdinarySharesThatAreAntidilutiveInPeriodPresented_60" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/LossPerShareDetailsNarrative" xlink:title="999115 - Disclosure - Loss per share (Details Narrative)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://fusion-fuel.eu/role/SubsequentEventsDetailsNarrative" xlink:title="999116 - Disclosure - Subsequent events (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_IfrsStatementLineItems" xlink:label="loc_HTOOIfrsStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:label="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesAxis" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesAxis" xlink:to="loc_ifrs-fullCategoriesOfRelatedPartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_RoyalUraniumIncMember" xlink:label="loc_HTOORoyalUraniumIncMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:to="loc_HTOORoyalUraniumIncMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_InvestorsMember" xlink:label="loc_HTOOInvestorsMember_110" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCategoriesOfRelatedPartiesDomain" xlink:to="loc_HTOOInvestorsMember_110" xlink:type="arc" order="111" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NonadjustingEventsAfterReportingPeriodAxis" xlink:label="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:to="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NonadjustingEventsAfterReportingPeriodDomain" xlink:label="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodAxis" xlink:to="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NonadjustingEventsAfterReportingPeriodDomain" xlink:label="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodAxis" xlink:to="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_NonadjustingEventsAfterReportingPeriodMember" xlink:label="loc_HTOONonadjustingEventsAfterReportingPeriodMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullNonadjustingEventsAfterReportingPeriodDomain" xlink:to="loc_HTOONonadjustingEventsAfterReportingPeriodMember_0" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalAxis" xlink:label="loc_ifrs-fullClassesOfShareCapitalAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:to="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="loc_ifrs-fullClassesOfShareCapitalDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullClassesOfShareCapitalAxis" xlink:to="loc_ifrs-fullClassesOfShareCapitalDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SeriesAConvertiblePreferredSharesMember" xlink:label="loc_HTOOSeriesAConvertiblePreferredSharesMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOSeriesAConvertiblePreferredSharesMember_0" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ClassAOrdinarySharesMember" xlink:label="loc_HTOOClassAOrdinarySharesMember_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOClassAOrdinarySharesMember_30" xlink:type="arc" order="33" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_PrefundedWarrantsMember" xlink:label="loc_HTOOPrefundedWarrantsMember_120" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_HTOOPrefundedWarrantsMember_120" xlink:type="arc" order="123" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_WarrantsMember" xlink:label="loc_ifrs-fullWarrantsMember_130" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullClassesOfShareCapitalDomain" xlink:to="loc_ifrs-fullWarrantsMember_130" xlink:type="arc" order="133" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsAxis" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsDomain" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SupplierFinanceArrangementsDomain" xlink:label="loc_ifrs-fullSupplierFinanceArrangementsDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsAxis" xlink:to="loc_ifrs-fullSupplierFinanceArrangementsDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ForbearanceAgreementMember" xlink:label="loc_HTOOForbearanceAgreementMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsDomain" xlink:to="loc_HTOOForbearanceAgreementMember_0" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_LetterAgreementMember" xlink:label="loc_HTOOLetterAgreementMember_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsDomain" xlink:to="loc_HTOOLetterAgreementMember_70" xlink:type="arc" order="72" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_SecuritiesPurchaseAgreementMember" xlink:label="loc_HTOOSecuritiesPurchaseAgreementMember_110" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullSupplierFinanceArrangementsDomain" xlink:to="loc_HTOOSecuritiesPurchaseAgreementMember_110" xlink:type="arc" order="114" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesAxis" xlink:label="loc_ifrs-fullCounterpartiesAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ifrs-fullDisclosureOfTransactionsBetweenRelatedPartiesTable" xlink:to="loc_ifrs-fullCounterpartiesAxis" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="loc_ifrs-fullCounterpartiesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_ifrs-fullCounterpartiesAxis" xlink:to="loc_ifrs-fullCounterpartiesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_BevilacquaPLLCMember" xlink:label="loc_HTOOBevilacquaPLLCMember_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ifrs-fullCounterpartiesDomain" xlink:to="loc_HTOOBevilacquaPLLCMember_70" xlink:type="arc" order="73" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ConvertiblePromissoryNotesDescription" xlink:label="loc_HTOOConvertiblePromissoryNotesDescription_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOConvertiblePromissoryNotesDescription_160" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ParValuePerShare" xlink:label="loc_ifrs-fullParValuePerShare_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullParValuePerShare_160" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfSharesOutstanding" xlink:label="loc_ifrs-fullNumberOfSharesOutstanding_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfSharesOutstanding_160" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfSharesIssued" xlink:label="loc_ifrs-fullNumberOfSharesIssued_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullNumberOfSharesIssued_160" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DescriptionOfAcquisitionOfEntityByMeansOfEquityIssue" xlink:label="loc_ifrs-fullDescriptionOfAcquisitionOfEntityByMeansOfEquityIssue_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullDescriptionOfAcquisitionOfEntityByMeansOfEquityIssue_160" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProfessionalFeesExpense" xlink:label="loc_ifrs-fullProfessionalFeesExpense_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullProfessionalFeesExpense_160" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_MaximumAggregateOfferingPrice" xlink:label="loc_HTOOMaximumAggregateOfferingPrice_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOMaximumAggregateOfferingPrice_160" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_DebtInstrumentOfOutstandingBalanceAmount" xlink:label="loc_HTOODebtInstrumentOfOutstandingBalanceAmount_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOODebtInstrumentOfOutstandingBalanceAmount_160" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_DebtInstrumentOfOutstandingBalancePrecentage" xlink:label="loc_HTOODebtInstrumentOfOutstandingBalancePrecentage_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOODebtInstrumentOfOutstandingBalancePrecentage_160" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ServicesExpense" xlink:label="loc_ifrs-fullServicesExpense_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullServicesExpense_160" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ServicesExpenseReducing" xlink:label="loc_HTOOServicesExpenseReducing_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOServicesExpenseReducing_160" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IssuedCapitalOrdinaryShares" xlink:label="loc_ifrs-fullIssuedCapitalOrdinaryShares_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_ifrs-fullIssuedCapitalOrdinaryShares_160" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_PercentageOfDiscountToMarket" xlink:label="loc_HTOOPercentageOfDiscountToMarket_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOPercentageOfDiscountToMarket_160" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_ExecutionMinimumPaymentAmount" xlink:label="loc_HTOOExecutionMinimumPaymentAmount_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOExecutionMinimumPaymentAmount_160" xlink:type="arc" order="13" />
      <link:loc xlink:type="locator" xlink:href="htoo-20260630.xsd#HTOO_AggregateGrossProceeds" xlink:label="loc_HTOOAggregateGrossProceeds_160" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_HTOOIfrsStatementLineItems" xlink:to="loc_HTOOAggregateGrossProceeds_160" xlink:type="arc" order="14" />
    </link:definitionLink>
</link:linkbase>
