SEC EDGAR · Insider purchase

IMSR: Pardee Charles G reported buying shares

1 reported purchase of non-derivative securities. Code P includes open-market and private purchases; it does not establish motive.

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Became public: Oct 15, 2026
Event / period date: Oct 14, 2026
Collected: Nov 8, 2026, 11:26 PM EST

Why this is connected

This filing identifies Terrestrial Energy Inc. /DE/ as the issuer of the security. The company's SEC identifier connects it to IMSR.

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Reported transactions

Pardee Charles G (Director)

7,500 shares · Common Stock

Oct 14, 2026 · $2.95 per share

Value calculated from the reported shares and prices: $22,125.00.

Code P covers open-market and private purchases. Grants, option exercises, and tax withholding are excluded from purchase results. Read the source footnotes for ownership and pricing details.

Source evidence

Read the source excerpt or record

<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0609</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2026-10-14</periodOfReport>

    <issuer>
        <issuerCik>0002019804</issuerCik>
        <issuerName>Terrestrial Energy Inc. /DE/</issuerName>
        <issuerTradingSymbol>IMSR</issuerTradingSymbol>
        <issuerForeignTradingSymbol></issuerForeignTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001544340</rptOwnerCik>
            <rptOwnerName>Pardee Charles G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerNonUSAddressFlag>false</rptOwnerNonUSAddressFlag>
            <rptOwnerStreet1>C/O TERRESTRIAL ENERGY INC.</rptOwnerStreet1>
            <rptOwnerStreet2>2730 W. TYVOLA ROAD, SUITE 100</rptOwnerStreet2>
            <rptOwnerCity>CHARLOTTE</rptOwnerCity>
            <rptOwnerState>NC</rptOwnerState>
            <rptOwnerZipCode>28217</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>true</isDirector>
            <isOfficer>false</isOfficer>
            <isTenPercentOwner>false</isTenPercentOwner>
            <isOther>false</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>false</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2026-10-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>false</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>7500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>2.95</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>14258</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <ownerSignature>
        <signatureName>/s/ Brian Thrasher, as Attorney-in-Fact</signatureName>
        <signatureDate>2026-10-15</signatureDate>
    </ownerSignature>
</ownershipDocument>

Original documents control. Public records can be amended, and company connections may require interpretation. Nothing here is investment advice.