C3is Inc美股招股说明书(2025-10-09版)

Table of Contents PROSPECTUS SUPPLEMENT (To Prospectus dated March6, 2025) 800,000 Common Shares C3IS INC. We are offering 800,000 of our shares of common stock, par value $0.01 per share, or common shares, directly to a number of institutional investors, or the Investors,pursuant to this prospectus supplement and the accompanying base prospectus. The offering price of the shares is $2.50 per share. Our common shares are listed on The Nasdaq Capital Market, or Nasdaq, under the symbol “CISS”. On October7, 2025, the closing price for our common shares asreported on the Nasdaq was $4.07 per share. The aggregate market value of our outstanding common shares held bynon-affiliateson October7, 2025 was $6,855,044based on 1,684,286 common shares issued and outstanding held bynon-affiliatesand a per share price of $4.07 based on the closing sale price of our common shares onOctober7, 2025 as reported by Nasdaq. Pursuant to General Instruction I.B.5 of FormF-3,in no event will we sell our securities in a public primary offering with a valueexceeding more thanone-thirdof our public float in any12-monthperiod so long as our public float remains below $75million. During the 12 calendar months prior toand including the date of this prospectus, we have not sold any securities pursuant to General Instruction I.B.5 of FormF-3. Investing in our common shares involves a high degree of risk and uncertainty. See “Risk Factors” beginning on pageS-8of thisprospectus supplement, and page 1 of the accompanying base prospectus, and in our annual report on Form20-Ffor the fiscalyear ended on December31, 2024, filed with the U.S. Securities and Exchange Commission, or the Commission, on April28, 2025,or our “Annual Report” which is incorporated by reference herein, to read about the risks you should consider before purchasingour common shares. We have retained Aegis Capital Corp. (whom we refer to herein as the Placement Agent) as our exclusive Placement Agent to use its reasonable best efforts to solicitoffers to purchase our common shares in this offering. The Placement Agent is not selling any of our common shares pursuant to this prospectus supplement or theaccompanying base prospectus. We expect that delivery of our common shares being offered pursuant to this prospectus supplement will be made to the Investors in theoffering on or about October9, 2025, subject to customary closing conditions. PerShareTotalPublic offering price$2.50$2,000,000.00Placement agent fees(1)$0.15$120,000Proceeds to the Company before expenses$2.35$1,880,000 (1)We have agreed to pay the Placement Agent a cash fee of 6.0%. See “Plan of Distribution” beginning on pageS-12of this prospectus supplement for additionaldisclosure regarding Placement Agent fees and estimated offering expenses. None of the Commission, any state securities commission, or any other regulatory body has approved or disapproved of these securities or passed on the adequacyor accuracy of this prospectus supplement or the accompanying base prospectus. Any representation to the contrary is a criminal offense. Sole Placement Agent Aegis Capital Corp. The date of this prospectus supplement is October8, 2025. Table of Contents TABLE OF CONTENTS PROSPECTUS SUMMARY ABOUT THIS PROSPECTUS SUPPLEMENTCAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTSENFORCEABILITY OF CIVIL LIABILITIESPROSPECTUS SUPPLEMENT SUMMARYTHE OFFERINGRISK FACTORSUSE OF PROCEEDSCAPITALIZATIONDILUTIONPLAN OF DISTRIBUTIONLEGAL MATTERSEXPERTSWHERE YOU CAN FIND ADDITIONAL INFORMATIONINFORMATION INCORPORATED BY REFERENCE BASE PROSPECTUS SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTSRISK FACTORSSERVICE OF PROCESS AND ENFORCEMENT OF LIABILITIESABOUT THIS PROSPECTUSPROSPECTUS SUMMARYWHERE YOU CAN FIND ADDITIONAL INFORMATIONINCORPORATION OF CERTAIN INFORMATION BY REFERENCEUSE OF PROCEEDSCAPITALIZATIONDESCRIPTION OF SECURITIES WE MAY OFFERDESCRIPTION OF PREFERRED STOCKDESCRIPTION OF DEBT SECURITIESDESCRIPTION OF WARRANTSDESCRIPTION OF PURCHASE CONTRACTSDESCRIPTION OF RIGHTSDESCRIPTION OF UNITSDESCRIPTION OF DEPOSITARY SHARESCONVERTIBLE OR EXCHANGEABLE SECURITIES DESCRIPTION OF CAPITAL STOCKFORM, EXCHANGE AND TRANSFERBOOK-ENTRY PROCEDURES AND SETTLEMENTPLAN OF DISTRIBUTIONLEGAL MATTERSEXPERTS Table of Contents ABOUT THIS PROSPECTUS SUPPLEMENT This prospectus supplement and the accompanying base prospectus are part of a registration statement that we filed with the Commission, utilizing a“shelf” registration process. This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering described herein and thesecurities offered hereby, and also adds to and updates information contained in the accompanying base prospectus and the documents incorporated byreference into this prospectus supplement and the accompanying base prospectus. The second part, the base prospectus, gives more general information about securities we may offer from time to