C3is Inc美股招股说明书(2026-02-25版)

C3IS INC. This is a supplement (“Prospectus Supplement”) to the prospectus, dated December18, 2025 (“Prospectus”), of C3is Inc. (the “Company”), whichforms a part of the Company’s Registration Statement onFormF-1(RegistrationNo.333-272939),as amended or supplemented from time to time. ThisProspectus Supplement is being filed to update and supplement the information included in the Prospectus with the information contained in our ReportonForm6-K,furnished to the U.S. Securities and Exchange Commission on February 25, 2026 (the“Form6-K”).Accordingly, we have attachedtheForm6-Kto this Prospectus Supplement. This Prospectus Supplement should be read in conjunction with, and delivered with, the Prospectus and is qualified by reference to the Prospectusexcept to the extent that the information in this Prospectus Supplement supersedes the information contained in the Prospectus. This ProspectusSupplement is not complete without, and may not be delivered or utilized except in connection with, the Prospectus, including any amendments orsupplements to it. Investing in our securities involves a high degree of risk. See “Risk Factors” beginning on page 8 of the Prospectusfor a discussion of information that should be considered in connection with an investment in our securities. Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities ordetermined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense. The date of this prospectus supplement is February 25, 2026. REPORT OF FOREIGN PRIVATE ISSUERPURSUANT TO RULE 13a-16 OR 15d-16UNDER THE SECURITIES EXCHANGE ACT OF 1934 For the month of February 2026 Commission File Number 001-41717 (Translation of registrant’s name into English) Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F. Form 20-F☒Form 40-F☐ INFORMATION CONTAINED IN THIS FORM 6-K REPORT C3is Inc. (the “Company”) has entered into an At-The-Market Issuance Sales Agreement with Aegis Capital Corp. (“Aegis”) dated February25, 2026(the “Sales Agreement”). Under the terms of the Sales Agreement, the Company may, from time to time, sell its common shares having an aggregateoffering value of up to $98million through Aegis, as sales agent. The Company will determine, at its sole discretion, the timing and number of shares tobe sold under the ATM facility. The Company intends to use the proceeds from the offering and sale of the securities for working capital and generalcorporate purposes, which may include payment of part of the purchase price of the two MR product tankers for which it has contracted or other vesselacquisitions. The offering and sale of the securities will be made pursuant to the Company’s previously filed registration statement on Form F-3 (FileNo.333-285135), declared effective on March6, 2025. The securities may be offered only by means of a prospectus, including a prospectus supplement,forming a part of such effective registration statement. Copies of the prospectus supplement and accompanying base prospectus related to the ATMoffering have been filed with the SEC and may be obtained from the SEC’s website at http://www.sec.gov. The foregoing descriptions of the material terms of the Sales Agreement are qualified in their entirety by reference to the full text of the SalesAgreement, which is filed as Exhibit 1.1 to this report on Form 6-K and incorporated herein by reference. This report on Form 6-K and the exhibits therefore, and the information contained herein and therein, do not constitute an offer to sell or the solicitationof offers to buy any securities of the Company, and shall not constitute an offer, solicitation or sale of any security in any state or jurisdiction in whichsuch offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state or jurisdiction. The legal opinion of Reeder& Simpson, P.C. relating to the validity of the securities being offered pursuant to the Sales Agreement is filed as Exhibit5.1 to this report on Form 6-K. ***** This report on Form 6-K, including Exhibits 1.1 and 5.1, are hereby incorporated by reference into the Company’s Registration Statement on Form S-8(Reg. No.333-273306) filed with the Securities and Exchange Commission on July18, 2023 and Registration Statement on Form F-3 (Reg. No.333-285135) filed with the Securities and Exchange Commission on February21, 2025. EXHIBIT INDEX At-The-Market Issuance Sales Agreement between C3is Inc. and Aegis Capital Corp. LLC dated February 25, 2026. SIGNATURES Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by theundersigned, thereunto duly authorized. Date: February25, 2026 C3IS INC. AT-THE-MARKET ISSUANCE SALES AGREEMENT February 25, 2026 Aegis Capital