C3is Inc美国招股说明书（2026年2月25日版）

PROSPECTUS SUPPLEMENT NO. 6(TO PROSPECTUS DATED DECEMBER18, 2025) C3IS INC. 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 Prospectus 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. 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 to 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 ATM 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 which 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- 1.1At-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. By:/s/ Nina PyndiahName:Nina Pyndiah AT-THE-MARKET ISSUANCE SALES AGREEMENT February 25, 2026 Aegis Capital Corp.1345 Avenue of the Americas, 27th Floor Ladies and Gentlemen: C3is Inc., a company incorporated under the laws of the Republic of the Marshall Islands (the “Company”), confirms its agreement (this “Agreement”)with Aegis Capital Corp. (“Sales Agent”), as follows: 1.Issuance and Sale of Shares. The Company agrees to issue and sell through or to Sales Agent, as its exclusive sales agent, shares of CommonStock, $0.01 par value per share (the “Common Stock” and such offered shares, the “Placement Shares”), from time to time during the term ofthis Agreement and on the terms set forth in this Agreement;provided however, that in no event will the Company issue or sell through Sales 1.1. Notwithstanding anything to the contrary contained herein, the parties hereto agree that compliance with the limitations set forth in thisSection1 on the amount of Placement Shares issued and sold under this Agreement will be the sole responsibility of the Company and that SalesAgent will have no obligation in connection with such compliance, provided that Sales Agent follows the lawful trading instructions provided bythe Company pursuant to any Placement Notice in all material respects. The issuance and sale of Placement Shares through Sales Agent will beeffected pursuant to the Registration Statement (as defined below) filed by the Company and declared effective by the U.S. Securities and 1.2. The Company has filed with the SEC, in accordance with the provisions of the Securities Act of 1933, as amended (the “Securities Act”), andthe rules and regulations thereunder (the “Securities Act Regulations”), a registration statement on Form F-3 (File No.333-285135), including abase prospectus, relating to certain securities, including the Placement Shares, to be issued from time to time by the Company, and whichincorporates by reference documents that the Company has filed or will file in accordance with the provisions of the Securities Exchange Act of