C3is Inc美股招股说明书(2026-08-19版)

C3IS INC. This is a supplement (“Prospectus Supplement”) to the prospectus, dated July27, 2026 (“Prospectus”), of C3is Inc. (the “Company”), whichforms a part of the Company’s Registration Statement onFormF-1(RegistrationNo.333-297627),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 August19, 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 13 of the Prospectus for a discussion ofinformation 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. REPORT OF FOREIGN PRIVATE ISSUERPURSUANT TO RULE 13a-16 OR 15d-16UNDER THE SECURITIES EXCHANGE ACT OF 1934 For the month of August 2026 Commission File Number 001-41717 (Translation of registrant’s name into English) INFORMATION CONTAINED IN THIS FORM 6-K REPORT Reverse Stock Split On August18, 2026, C3is Inc. (the “Company”) filed an amendment to its Restated Articles of Incorporation, to effectuate a reverse stock split ofthe Company’s issued and outstanding shares of common stock, par value of $0.01 per share. A copy of the amendment is attached hereto as Exhibit 4.1. Effective as of 11:59 p.m. Eastern time on August18, 2026, the Company effected a one-for-40 reverse stock split of its shares of common stock,and the Company’s common stock began trading on a split-adjusted basis on The Nasdaq Capital Market as of the opening of trading on August19,2026. The reverse stock split reduced the number of outstanding shares of the Company’s common stock from approximately 57.6million shares toapproximately 1.44million shares and affected all outstanding shares of common stock. No fractional shares were issued in connection with the reversesplit. Stockholders who would otherwise hold a fractional share of the Company’s common stock received a cash payment in lieu of such fractionalshare. The Company’s outstanding warrants and Series A Convertible Preferred Stock will be proportionately adjusted to increase the exercise price andreduce the number of shares issuable upon exercise to reflect the reverse stock split. The exercise price of the ClassB-1, B-2, C-1 and C-2 Warrants, and accordingly the conversion price of the Series A Convertible Preferred Stock,will be further adjusted to the lowest daily volume weighted average price during the period commencing five consecutive trading days immediatelypreceding and the five consecutive trading days following the effective time of the reverse split (if lower than the then current exercise price), with theClassB-1 and C-1 warrants also being exercisable pursuant to an alternative zero cash exercise exchange option, and the number of shares of commonstock issuable upon exercise will be proportionately adjusted such that the aggregate exercise price of such warrants as of their original issuance datewill remain unchanged. The par value and other terms of the Company’s shares of common stock were not affected by the reverse stock split. The Company’s post-reversesplit common shares have a new CUSIP number, Y18284409, and continue to trade on the Nasdaq Capital Market under the symbol “CISS”. A copy of the new form of stock certificate for the Company’s post-reverse stock split shares of common stock is attached hereto as Exhibit 4.2. ***** 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. EXHIBIT INDEX 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: August19, 2026 C3IS INC. ARTICLES OF AMENDMENT TO AMENDED AND RESTATED ARTICLES OF INCORPORATION OF C3IS INC. Under Section90 of theRepublic of the Marshall Islands Business C