Terrestrial Energy Inc美股招股说明书(2026-03-30版)

SUPPLEMENT TOPROSPECTUS FOR PRIMARY OFFERING OF UP TO18,776,119 SHARES OF COMMON STOCKAND SECONDARY OFFERING OF18,792,599 SHARES OF COMMON STOCK1,267,599 WARRANTS TO PURCHASE SHARES OF COMMON STOCKOF TERRESTRIAL ENERGY INC. This Supplement No. 1 (this “Supplement No. 1”) updates, amends and supplements the prospectus datedDecember 30, 2025 (as supplemented or amended from time to time, the “Prospectus”), which forms a partof our Registration Statement on FormS-1(RegistrationNo.333-291796, the “Form S-1”). Capitalizedterms used in this prospectus supplement and not otherwise defined herein have the meanings specified inthe Prospectus. This Supplement No. 1 is being filed to update, amend and supplement the information included in theProspectus with information contained in our Annual Report on Form 10-K filed with the SEC on March30, 2026, which is set forth below. To the extent information in this Supplement No. 1 differs from,updates or conflicts with information contained in the Prospectus, the information in this Supplement No.1 is the more current information. This Supplement No. 1 is not complete without the Prospectus. ThisSupplement No. 1 should be read in conjunction with the Prospectus, which is to be delivered with thisSupplement No. 1, and is qualified by reference thereto, except to the extent that the information in thisSupplement No. 1 updates or supersedes the information contained in the Prospectus. Please keep thisprospectus supplement with the Prospectus for future reference. Investing in our securities involves a high degree of risk. See “Risk Factors” beginning on page 12 ofthe Prospectus. NEITHERTHE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATESECURITIESREGULATORY AGENCY HAS APPROVED OR DISAPPROVED OF THESECURITIES DESCRIBED IN THE PROSPECTUS OR THIS PROSPECTUS SUPPLEMENT ORPASSEDUPON THE ADEQUACY OR ACCURACY OF THE DISCLOSURE IN THEPROSPECTUS OR THIS PROSPECTUS SUPPLEMENT. ANY REPRESENTATION TO THECONTRARY CONSTITUTES A CRIMINAL OFFENSE. The date of this prospectus supplement is March 30, 2026. Table of Contents (Mark One)☒ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGEACT OF 1934For the fiscal year ended December 31, 2025or☐TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIESEXCHANGE ACT OF 1934001-42252(Commission File Number)TERRESTRIAL ENERGY INC.(Exact name of registrant as specified in its charter) Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule405 of the Securities Act. Yes☐No☒ Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or 15(d) of the Act. Yes☐No☒ Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of theSecurities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to filesuch reports), and (2) has been subject to such filing requirements for the past 90 days. Yes☒No☐ Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to besubmitted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such shorter period that the registrant wasrequired to submit such files). Yes☒No☐ Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, asmaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,”“smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act: Large accelerated filer☐Accelerated filer☐Non-accelerated filer☒Smaller reporting company☒Emerging growth company☒ If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transitionperiod for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the ExchangeAct.☐ Indicate by check mark whether the registrant has filed a report on and attestation to its management’s assessment of theeffectiveness of its internal control over financial reporting under Section 404(b) of the Sarbanes-Oxley Act by the registered publicaccounting firm that prepared or issued its audit report.☐ If securities are registered pursuant to Section 12(b) of the Act, indicate by check mark whether the financial statements ofthe registrant included in the filing reflect the correction of an error to previously issued financial statements.☐ Indicate by check mark whether any of those error corrections are restatements that required a recovery analysis ofincentive-based compensation received by any of the registrant’s executive officers during the relevant recovery period pursuant to§240.10D-1(b).☐ Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Act). Yes☐No☒ The aggregate market value of the com