Terrestrial Energy Announces Secondary Offering Of Up To 2,263,805 Shares Of Common Stock

TERRESTRIAL ENERGY INC.

Secondary Offering of up to

2,263,805 SHARES OF COMMON STOCK

This prospectus relates to the resale by the selling stockholder identified herein (the "Selling Stockholder") of up to 2,263,805 shares (the "Shares") of the common stock, par value $0.0001 per share (the "Common Stock") of Terrestrial Energy Inc., a Delaware corporation ("us," "we," "Terrestrial Energy," or the "Company"). The Shares are issuable upon the exercise of stock options awarded to the Selling Stockholder during his employment with Terrestrial Energy Development, Inc. ("TEDI") before its business combination with the Company.

The Selling Stockholder may offer all or part of the Shares for resale from time to time through public or private transactions, at either prevailing market prices or at privately negotiated prices. The Shares are being registered to permit the Selling Stockholder to sell Shares from time to time, in amounts, at prices and on terms determined at the time of offering. The Selling Stockholder may sell the Shares through ordinary brokerage transactions, in underwritten offerings, directly to market makers of our securities or through any other means described in the section entitled "Plan of Distribution" herein. In connection with any sales of Shares offered hereunder, the Selling Stockholder, any underwriters, agents, brokers or dealers participating in such sales may be deemed to be "underwriters" within the meaning of the Securities Act, or the Exchange Act. We are registering the Shares for resale by the Selling Stockholder, or his donees, pledgees, transferees, distributes or other successors-in-interest selling our Common Stock or interests in our Common Stock received after the date of this prospectus from the Selling Stockholder as a gift, pledge, partnership distribution or other transfer.

As October 26, 2026, we had 105,790,799 shares of Common Stock and share equivalents outstanding, including 23,216,699 shares of Common Stock that would be issued and outstanding upon the exchange of outstanding Exchangeable Shares.

We will not receive any proceeds from the sale of the Shares by the Selling Stockholder. We will bear all costs, expenses, and fees in connection with the registration of the Shares. The Selling Stockholder will bear all commissions and discounts, if any, attributable to any sales of the Shares. We will receive the proceeds from any exercise of the Selling Stockholder’s stock option awards. Assuming the exercise in full of all such stock options for cash, we would receive aggregate proceeds of approximately $1.6 million.