C3is Inc美股招股说明书(2026-01-20版)

C3IS INC. This is a supplement (“Prospectus Supplement”) to the prospectus, dated December11, 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 ReportsonForm6-K,furnished to the U.S. Securities and Exchange Commission on January 20, 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 14 of theProspectus for 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 January20, 2026. REPORT OF FOREIGN PRIVATE ISSUERPURSUANT TO RULE13a-16OR15d-16UNDER THE SECURITIES EXCHANGE ACT OF 1934 For the month of January 2026 Commission File Number001-41717 (Translation of registrant’s name into English) Indicate by check mark whether the registrant files or will file annual reports under cover of Form20-For Form40-F. Form20-F☒Form40-F☐ INFORMATION CONTAINED IN THIS FORM6-KREPORT Vessel Acquisition Agreement C3is Inc. (the “Company”) has entered into an agreement to acquire a 2011 South Korean-built product tanker with 47,203 deadweight tons (dwt)capacity for a purchase price of $22.9million, from an entity affiliated with Brave Maritime Corp Inc. The transaction with Brave Maritime, which isaffiliated with members of the Vafias family, was approved by the independent directors of the Company, which obtained independent valuations of thevessel, and used the average price of these valuations. The vessel is expected to be delivered between Q1 and Q2 2026. The Company has the option to pay for the vessel one year after the date of thepurchase agreement, without interest. Following this vessel delivery, and assuming no further acquisitions or dispositions other than the pending product tanker acquisition previouslyannounced, the Company’s fleet will total 6 vessels, three Handysize drybulk carriers with a total capacity of 97,664 dwt, an Aframax oil tanker with acapacity of approximately 115,800 dwt and two product tankers with a capacity of approximately 97,203 dwt, resulting in a fleet total capacity of310,667 dwt. ***** 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 July 18, 2023 and Registration Statement on Form F-3 (Reg. No. 333- 285135) filed with the Securities andExchange Commission on February 21, 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: January20, 2026 C3IS INC.