C3is Inc美股招股说明书(2025-12-19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 December12, 2025 and December19,2025(the“Form6-Ks”).Accordingly, we have attached theForm6-Ksto 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 December19, 2025. REPORT OF FOREIGN PRIVATE ISSUERPURSUANT TO RULE13a-16OR15d-16UNDER THE SECURITIES EXCHANGE ACT OF 1934 For the month of December 2025 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 On December12, 2025, C3is Inc. (the “Company”) completed a registered offering (the “Offering”) of 7,500,000 units, consisting of (a)1,700,000 common units (“Common Units”), each Common Unit consisting of one common share, par value $0.01 per share, of the Company(“Common Share”),oneClass DWarrantto purchase one Common Share or otherwise such greater number of Common Shares, up to 26,162,790, asdetermined under the automatic adjustment feature of the ClassD Warrant (“ClassDWarrant”), and one ClassE Warrant exercisable for that number ofCommon Shares, up to 18,662,790, determined in accordance with the adjustment formula of the ClassE Warrant (“ClassE Warrant”), and (b)5,800,000 pre-fundedunits(“Pre-FundedUnits”), eachPre-FundedUnit consisting of onepre-fundedwarrant to purchase one Common Share at anexercise price of $0.00001 per Common Share,oneClass DWarrant and oneClass EWarrant. The aggregate gross proceeds to the Company from the Offering, before deducting placement agent fees and other expenses payable by theCompany, were approximately $9.0million. Attached to this report on Form6-Kas Exhibit 1.1 is a copy of the Placement Agent Agreement, dated December11, 2025, by and between theCompany and Aegis Capital Corp. Attached to this report on Form6-Kas Exhibit 4.1 is a copy of the form ofClass DWarrant. Attached to this report on Form6-Kas Exhibit 4.2 is a copy of the form ofClass EWarrant. Attached to this report on Form6-Kas Exhibit 4.3 is a copy of the form of Pre-funded Warrant. Each ClassD Warrant is exercisable upon issuance and expires five years after the issuance date. The initial exercise price of the ClassD Warrantsis $1.20. On the 10th trading day following the closing of this offering, the exercise price of the outstanding ClassD Warrants will automatically reset tothe “Adjustment Price,” which shall be the greater of (i)the Floor Price (as defined below) based on the Nasdaq Minimum Price (as defined in NasdaqListing Rule 5635(d)(1)(A), which is as the lower of the most recent Nasdaq Official Closing Price for the Common Shares and the average of theNasdaq Official Closing Price of the Common Shares over the last five trading days) immediately preceding the pricing of this offering, which equals$0.344, or (ii)the lesser of (x)the then exercise price and (y)the lowest daily volume-weighted average price (“VWAP”) of the Common Shares duringsuch 10 trading days period (the “Adjustment Period”), and the number of Common Shares underlying the then-outstanding ClassD Warrants will beproportionally increased such that the then aggregate exercise price of all such ClassD Warrants based on the Adjustment Price, equals the aggregateexercise price of all such ClassD Warrants on the original issuance date that remain outstanding. The “Floor Price” equals 20% of the most recentNasdaq Minimum Price of the Common Shares immediately prior to the applicable date of determination. Five trading days after the six-monthan