CISSV C3is Inc

Filed Pursuant to Rule 424(b)(3)

Registration No. 333-272939

PROSPECTUS SUPPLEMENT NO. 1

(TO PROSPECTUS DATED MAY 8, 2024)

C3IS INC.

This is a supplement (“Prospectus Supplement”) to the prospectus, dated May 8, 2024 (“Prospectus”), of C3is Inc. (the “Company”), which forms a part of the Company’s Registration Statement on Form F-1 (Registration No. 333-272939), as amended or supplemented from time to time. This Prospectus Supplement is being filed to update and supplement the information included in the Prospectus with the information contained in our Report on Form 6-K, furnished to the U.S. Securities and Exchange Commission on May 28, 2024 (the “Form 6-K”). Accordingly, we have attached the Form 6-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 Prospectus except to the extent that the information in this Prospectus Supplement supersedes the information contained in the Prospectus. This Prospectus Supplement is not complete without, and may not be delivered or utilized except in connection with, the Prospectus, including any amendments or supplements to it.

Investing in our securities involves a high degree of risk. See “Risk Factors” beginning on page 15 of the Prospectus 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 or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is May 28, 2024.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

REPORT OF FOREIGN PRIVATE ISSUER

PURSUANT TO RULE 13a-16 OR 15d-16

UNDER THE SECURITIES EXCHANGE ACT OF 1934

For the month of May 2024

Commission File Number 001-41717

C3IS INC.

(Translation of registrant’s name into English)

331 Kifissias Avenue Erithrea 14561 Athens, Greece

(Address of principal executive office)

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

Financial and Operating Results for the three months ended March 31, 2024

Attached to this report on Form 6-K as Exhibit 99.1 is a copy of C3is Inc.’s (the “Company”) financial and operating results for the quarter ended March 31, 2024.

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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 with the Securities and Exchange Commission on July 18, 2023.

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 the undersigned, thereunto duly authorized.

Date: May 28, 2024

Exhibit 99.1

May 28, 2024

On May 28, 2024, C3is Inc. (the “Company”) announced its financial and operating results for the three months ended March 31, 2024.

First Quarter 2024 Results:

Delivery of Drybulk Carrier

In April 2024, our Company announced an agreement to acquire a 2012-built Japanese handysize drybulk carrier from an affiliated company. Following this vessel acquisition and the delivery of the 33,664 DWT handysize drybulk carrier to our Company in May 2024, the total fleet capacity increased to 213,468 dwt. 10% of the purchase price was paid on delivery, with the remaining 90% due in April 2025.

Forward-Looking Statements

Matters discussed in this report may constitute forward-looking statements. Forward-looking statements reflect our current views with respect to future events and financial performance and may include statements concerning plans, objectives, goals, strategies, future events or performance and underlying assumptions and other statements, which are other than statements of historical facts. The forward-looking statements in this report are based upon various assumptions, many of which are based, in turn, upon further assumptions, including without limitation, management’s examination of historical operating trends, data contained in our records and other data available from third parties. Although C3is Inc. believes that these assumptions were reasonable when made, because these assumptions are inherently subject to significant uncertainties and contingencies which are difficult or impossible to predict and are beyond our control, C3is Inc. cannot assure you that it will achieve or accomplish these expectations, beliefs or projections. Important factors that, in our view, could cause actual results to differ materially from those discussed in the forward-looking statements include risks discussed in our filings with the SEC and the following: the strength of world economies and

currencies, general market conditions, including changes in charter hire rates and vessel values, charter counterparty performance, changes in demand that may affect attitudes of time charterers to scheduled and unscheduled drydockings, shipyard performance, changes in C3is Inc.’s operating expenses, including bunker prices, drydocking and insurance costs, ability to fund the remaining purchase price for certain of our vessels, ability to obtain financing and comply with covenants in our financing arrangements, or actions taken by regulatory authorities, potential liability from pending or future litigation, domestic and international political conditions, the conflict in Ukraine and related sanctions, the conflict in Israel and Gaza, potential disruption of shipping routes due to ongoing attacks by Houthis in the Red Sea and Gulf of Aden or accidents and political events or acts by terrorists.

Certain shipping industry information, statistics and charts contained herein have been derived from industry sources. You are hereby advised that such information, statistics and charts have not been prepared specifically for inclusion in this presentation and the Company has not undertaken any independent investigation to confirm the accuracy or completeness of such information.

Readers of this presentation should review our filings with the SEC for a discussion of factors and circumstances that could affect our future financial results and our ability to realize the expectations stated herein. EBITDA, Adjusted EBITDA, Time Charter Equivalent Revenues and Time Charter Equivalent $/per day may be included in our presentations, which are presented because they are used by management and certain investors to measure a company’s financial performance and underlying trends as they exclude certain items impacting overall comparability. EBITDA, Adjusted EBITDA, Time Charter Equivalent Revenues and Time Charter Equivalent $/per day are “non-GAAP financial measures” and should not be considered a substitute for net income or revenues in accordance with accounting principles generally accepted in the United States or as a measure of profitability or liquidity.

Fleet Data:

The following key indicators highlight the Company’s operating performance during the periods ended March 31, 2023 and March 31, 2024.

1) Average number of vessels is the number of owned vessels that constituted our fleet for the relevant period, as measured by the sum of the number of days each vessel was a part of our fleet during the period divided by the number of calendar days in that period.

2) Total calendar days for fleet are the total days the vessels we operated were in our possession for the relevant period including off-hire days associated with repairs, drydockings or special or intermediate surveys.

3) Total voyage days for fleet reflect the total days the vessels we operated were in our possession for the relevant period net of off-hire days associated with repairs, drydockings or special or intermediate surveys.

4) Fleet utilization is the percentage of time that our vessels were available for revenue generating voyage days, and is determined by dividing voyage days by fleet calendar days for the relevant period.

5) Total charter days for fleet are the number of voyage days the vessels operated on time or bareboat charters for the relevant period.

6) Total spot market charter days for fleet are the number of voyage days the vessels operated on spot market charters for the relevant period.

7) Fleet operational utilization is the percentage of time that our vessels generated revenue, and is determined by dividing voyage days excluding commercially idle days by fleet calendar days for the relevant period.

Reconciliation of EBITDA:

EBITDA represents net income before interest and finance costs, interest income and depreciation. EBITDA is not a recognized measurement under U.S. GAAP. Our calculation of EBITDA may not be comparable to that reported by other companies in the shipping or other industries.

EBITDA is included herein because it is a basis, upon which we and our investors assess our financial performance. It allows us to present our performance from period to period on a comparable basis and provides investors with a means of better evaluating and understanding our operating performance.

Reconciliation of TCE:

Time Charter Equivalent rate or “TCE” rate is determined by dividing revenue net of voyage expenses by voyage days for the relevant time period. TCE is a non-GAAP measure which provides additional meaningful information in conjunction with revenues, the most directly comparable GAAP measure to Time charter equivalent revenues assists the Company’s management in making decisions regarding the deployment and use of its vessels and in evaluating their financial performance. TCE is also a standard shipping industry performance measure, useful to the investors, as it is used primarily to compare period-to-period changes in a shipping company’s performance despite changes in the mix of charter types (i.e., spot charters or time charters, but not bareboat charters) under which the vessels may be employed between the periods and compare two shipping companies despite the charter types of their vessels.

C3is Inc.

Unaudited Condensed Consolidated Statements of Income

(Expressed in United States Dollars, except for number of shares)

C3is Inc.

Unaudited Condensed Consolidated Balance Sheets

(Expressed in United States Dollars)

C3is Inc.

Unaudited Condensed Consolidated Statements of Cash Flows

(Expressed in United States Dollars)