C3is Inc美股招股说明书(2026-07-28版)

C3IS INC. This is a supplement (“Prospectus Supplement”) to the prospectus, dated May13, 2026 (“Prospectus”), of C3is Inc. (the “Company”), whichforms a part of the Company’s Registration Statement on FormF-1 (Registration Nos. 333-276430 and 333-276597), as amended or supplemented fromtime to time. This Prospectus Supplement is being filed to update and supplement the information included in the Prospectus with the informationcontained in our Reports on Form6-K, furnished to the U.S. Securities and Exchange Commission on July22, 2026 and July28, 2026 (the“Forms6-K”).Accordingly, we have attached the Forms6-K to 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 July28, 2026. REPORT OF FOREIGN PRIVATE ISSUERPURSUANT TO RULE 13a-16 OR 15d-16UNDER THE SECURITIES EXCHANGE ACT OF 1934 For the month of July 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 2026 Annual Meeting of Stockholders On July17, 2026, C3is Inc. (the “Company”) held its 2026 Annual Meeting of Stockholders (the “Annual Meeting”). At the Annual Meeting,Mr.HarryN. Vafias and Mr.George Xiradakis were each re-elected as a ClassIII director for a three-year term expiring at the annual meeting of ourstockholders in 2029. Our stockholders also ratified the appointment of Deloitte Certified Public Accountants, S.A. as our independent auditors andapproved one or more amendments to our Restated Articles of Incorporation, as amended, to effect one or more reverse stock splits of our issued andoutstanding shares of common stock, at a ratio of not less than one-for-two and not more than one-for-1,000 and in the aggregate of not more thanone-for-1,000, inclusive, with the exact ratio to be determined by our Board of Directors in its discretion; provided each such reverse stock split iseffected within three years of such approval. EXHIBIT INDEX 99.1EPS Disclosure. This report on Form 6-K is hereby incorporated by reference into the Company’s Registration Statement on Form S-8 (Reg. No.333-273306) filed withthe Securities and Exchange Commission on July18, 2023 and Registration Statement on Form F-3 (Reg. No.333- 285135) filed with the Securities andExchange Commission on February21, 2025. 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: July22, 2026 C3IS INC. REVERSE STOCK SPLIT DISCLOSURE On April26, 2026, C3is Inc. (the “Company”) effected a one-for-seven (1-for-7) reverse split of its common shares (“Common Shares”). Thereverse stock split reduced the number of the Company’s outstanding Common Shares from approximately 3.8million shares to approximately 541,082shares and affected all issued and outstanding Common Shares. No fractional shares were issued in connection to the reverse split. Stockholders whowould otherwise hold a fractional share of the Company’s Common Shares received a cash payment in lieu of such fractional share. The par value andother terms of the Company’s Common Shares were not affected by the reverse stock split. All share and earnings per share information have been retroactively adjusted to reflect the stock split and the incremental reduction in theaggregate par value of all issued and outstanding Common Shares of $5,655 has been reflected as a reduction to “Capital stock” and a correspondingincrease in “Additional paid-in capital” on the Company’s balance sheet. The effect of the reverse stock split on per share amounts and weighted averagenumber of Common Shares outstanding for each of the three fiscal years ended December31, 2025 are as follows. REPORT OF FOREIGN PRIVATE ISSUERPURSUANT TO RULE 13a-16 OR 15d-16UNDER THE SECURITIES EXCHANGE ACT OF 1934 For the month of July 2026 Commission File Number 001-4171