Terrestrial Energy Inc. filed Post-Effective Amendment No. 3 to its Form S-1 Registration Statement on Form S-3 with the U.S. Securities and Exchange Commission on November 3, 2026, converting its original registration statements to Form S-3 format and covering a primary offering of up to 53,325,182 shares and a secondary offering of up to 14,887,245 shares of common stock.

Key Highlights

  • Primary offering size: Up to 53,325,182 shares of common stock underlying warrants and exchangeable shares are covered by this filing.
  • Secondary offering: Selling securityholders may resell up to 14,887,245 shares of common stock and 1,267,599 warrants to purchase shares of common stock.
  • No new securities registered: According to the filing, "No additional securities are being registered under this Post-Effective Amendment."
  • Conversion purpose: The amendment is being filed to convert each of the original registration statements into a Registration Statement on Form S-3.
  • Exchangeable shares component: Up to 23,216,699 shares are issuable upon conversion of Class B and Class C exchangeable shares of Terrestrial Energy Canada (Exchange) Inc.
  • Company classification: Terrestrial Energy is identified in the filing as a non-accelerated filer, smaller reporting company, and emerging growth company.

Terrestrial Energy's Post-Effective Amendment No. 3: Structure and Share Breakdown

Terrestrial Energy Inc., a Delaware corporation headquartered at 2730 W. Tyvola Road, Suite 100, Charlotte, NC 28217, filed Post-Effective Amendment No. 3 to its Form S-1 on Form S-3 (Registration No. 333-291796) on November 3, 2026. According to the filing, the amendment is being submitted primarily to convert each of the original registration statements into a Registration Statement on Form S-3. The original Form S-1 registration statement was declared effective by the SEC on December 23, 2025, and a prior Form S-4 registration statement (File No. 333-288735) was declared effective on September 26, 2025.

The primary offering covers shares issuable upon exercise of several categories of warrants and upon conversion of exchangeable shares. These include 4,275,000 shares tied to warrants issued to HCM Investor Holdings II, LLC (identified as the "Sponsor"), 2,575,000 shares tied to warrants issued to Cantor Fitzgerald and Co., 1,267,599 shares tied to working capital warrants, 10,490,884 shares tied to Legacy Terrestrial Warrants held by pre-Business Combination stockholders of Terrestrial Energy Development Inc., 11,500,000 shares tied to publicly traded warrants, and 23,216,699 shares issuable upon conversion of Class B and Class C exchangeable shares of Terrestrial Energy Canada (Exchange) Inc.

The filing notes that Post-Effective Amendment No. 2, filed and declared effective on June 5, 2026, combined prospectuses from the original registration statements and converted the prior Form S-4 into a Form S-1. The current Post-Effective Amendment No. 3 builds on that by converting the combined registration to Form S-3. The prospectus also covers resale by selling securityholders entitled to registration rights under the Amended and Restated Registration Rights Agreement, dated October 28, 2025. The filing states that "all applicable registration fees were paid at the time of the original filing of the registration statement."

What Was Disclosed

Terrestrial Energy Inc. filed a post-effective amendment to convert its registration statements to Form S-3, covering up to 53,325,182 primary shares and up to 14,887,245 secondary shares for resale, with no new securities added to the registration.