CISS C3is Inc

Filed Pursuant to Rule 424(b)(3)

Registration Nos. 333-276430 and 333-276597

PROSPECTUS SUPPLEMENT NO. 1

(TO PROSPECTUS DATED DECEMBER 13, 2024)

C3IS INC.

This is a supplement (“Prospectus Supplement”) to the prospectus, dated December 13, 2024 (“Prospectus”), of C3is Inc. (the “Company”), which forms a part of the Company’s Registration Statement on Form F-1 (Registration Nos. 333-276430 and 333-276597), 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 January 2, 2025 (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 9 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 January 2, 2025.

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 January 2025

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

Reverse Stock Split

On December 31, 2024, C3is Inc. (the “Company”) filed an amendment to its Restated Articles of Incorporation, to effectuate a reverse stock split of the 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 December 31, 2024, the Company effected a one-for-2.5 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 January 2, 2025. The reverse stock split reduced the number of outstanding shares of the Company’s common stock from approximately 10.6 million to approximately 4.2 million and affected all outstanding shares of common stock. No fractional shares were issued in connection to the reverse split. Stockholders who would otherwise hold a fractional share of the Company’s common stock received a cash payment in lieu of such fractional share. The Company’s outstanding warrants and Series A Convertible Preferred Stock will be proportionately adjusted to increase the exercise price and reduce the number of shares issuable upon exercise to reflect the reverse stock split.

The exercise price of the Class B-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 immediately preceding and the five consecutive trading days following the effective time of the reverse split (if lower than the then current exercise price), with the Class B-1 and C-1 warrants also being exercisable pursuant to an alternative cashless exercise exchange option, and the number of shares of common stock issuable upon exercise will be proportionately adjusted such that the aggregate exercise price of such warrants as of their original issuance date will 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-reverse split common shares have a new CUSIP number, Y18284 151, 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.

Attached to this report on Form 6-K as Exhibit 99.1 is a copy of the press release dated December 30, 2024 titled “C3is Inc. Announces Reverse Stock Split”.

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 the undersigned, thereunto duly authorized.

Date: January 2, 2025

Exhibit 4.1

ARTICLES OF AMENDMENT

TO

AMENDED AND RESTATED ARTICLES OF INCORPORATION

OF

C3IS INC.

Under Section 90 of the

Republic of the Marshall Islands Business Corporations Act

C3IS INC., a corporation incorporated in and existing under the law of the Republic of the Marshall Islands (the “Corporation”), hereby certifies as follows:

(a) The name of the Corporation is: C3IS INC.

(b) The Corporation was originally incorporated by filing Articles of Incorporation in the Republic of The Marshall Islands on July 25, 2022. Amended and Restated Articles of Incorporation were filed with the Registrar of Corporations of the Republic of the Marshall Islands on June 13, 2023, as amended by the Articles of Amendment filed with the Registrar of Corporations of the Republic of the Marshall Islands on April 11, 2024 (as so amended, restated and corrected, the “Articles of Incorporation”). A Statement of Designation was filed pursuant to Section 35(5) of the Business Corporations Act on June 20, 2023 in respect to the rights, preferences and privileges of the 5.00% Series A Cumulative Convertible Perpetual Preferred Stock of the Corporation.

(c) The Amended and Restated Articles of Incorporation are hereby amended by inserting the following as a new paragraph into Section FOURTH immediately following the last paragraph therein:

(e) Reverse Stock Split. As of 11:59 p.m. Eastern time on December 31, 2024 (the “Reverse Stock Split Effective Date”), each two-and-a-half (2.5) shares of Common Stock issued and outstanding immediately prior to the Reverse Stock Split Effective Date either issued and outstanding or held by the Corporation as treasury stock shall be combined into one (1) validly issued, fully paid and non-assessable share of Common Stock without any further action by the Corporation or the holder thereof (the “Reverse Stock Split”); provided that no fractional shares shall be issued to any holder and that in lieu of issuing any such fractional shares, fractional shares resulting from the Reverse Stock Split will be rounded down to the nearest whole share and provided, further, that stockholders who would otherwise be entitled to receive fractional shares because they hold a number of shares not evenly divisible by the ratio of the Reverse Stock Split will receive a cash payment (without interest and subject to applicable withholding taxes) in an amount per share equal to the closing price per share of Common Stock on the Nasdaq Stock Market on the trading day immediately preceding the Reverse Stock Split Effective Date, as adjusted for the reverse stock split as appropriate. Each certificate, if any, that immediately prior to the Reverse Stock Split Effective Date represented shares of Common Stock (“Old Certificates”), shall thereafter represent that number of shares of Common Stock into which the shares of Common Stock represented by the Old Certificate shall have been combined, subject to the elimination of fractional shares as described above. The reverse stock split described in this paragraph shall not change the number of shares of Common Stock authorized to be issued or the par value of the Common Stock. No change was made to the number of registered shares of Preferred Stock the Corporation is authorized to issue or to the par value of the Preferred Stock.

(d) This amendment to the Amended and Restated Articles of Incorporation was authorized by actions of the Board of Directors and shareholders of the Corporation as required by the BCA.

IN WITNESS WHEREOF, the Corporation has caused these Articles of Amendment to Amended and Restated Articles of Incorporation to be signed as of the 31st day of December, 2024, by its Chief Executive Officer, who hereby affirms and acknowledges, under penalty of perjury, that these Articles of Amendment are the act and deed of the Corporation and that the facts stated herein are true.

Exhibit 4.2

COMMON STOCK

CUSIP No. Y18284 151

C3IS INC.

FORMED UNDER THE LAWS OF THE REPUBLIC OF THE MARSHALL ISLANDS

THIS CERTIFIES THAT _______________________________________

is the owner of _______________________________________________

FULLY-PAID AND NON-ASSESSABLE SHARES OF COMMON STOCK, PAR VALUE $0.01 PER SHARE, OF

C3is Inc. (hereinafter called the “Company”), transferable on the books of the Company by the holder hereof in person or by duly authorized attorney upon surrender of this Certificate properly endorsed. This Certificate and the shares represented hereby are issued and shall be held subject to all the provisions of the Articles of Incorporation, as amended, and Bylaws of the Corporation, to all of which each holder, by acceptance hereof, assents.

Witness the signatures of the duly authorized officers of the Company.

Dated      , 20               [Affix Corporate Seal]

COUNTERSIGNED AND REGISTERED

EQUINITI TRUST COMPANY, LLC

TRANSFER AGENT AND REGISTRAR

C3IS INC. WILL FURNISH WITHOUT CHARGE TO EACH SHAREHOLDER WHO SO REQUESTS, A FULL STATEMENT OF THE DESIGNATION, RELATIVE RIGHTS, PREFERENCES AND LIMITATIONS OF EACH CLASS OF SHARES OF THE COMPANY AUTHORIZED TO BE ISSUED AND THE DESIGNATION, RELATIVE RIGHTS, PREFERENCES AND LIMITATIONS OF EACH SERIES, WHICH ARE FIXED AND THE AUTHORITY OF THE BOARD OF DIRECTORS TO DESIGNATE AND FIX THE RELATIVE RIGHTS, PREFERENCES AND LIMITATIONS OF OTHER SERIES. SUCH REQUEST MAY BE MADE TO THE OFFICE OF THE SECRETARY OF THE COMPANY OR TO THE TRANSFER AGENT. THE BOARD OF DIRECTORS MAY REQUIRE THE OWNER OF A LOST OR DESTROYED SHARES CERTIFICATE, OR HIS LEGAL REPRESENTATIVES, TO GIVE THE COMPANY A BOND TO INDEMNIFY IT AND ITS TRANSFER AGENTS AND REGISTRARS AGAINST ANY CLAIM THAT MAY BE MADE AGAINST THEM ON ACCOUNT OF THE ALLEGED LOSS OR DESTRUCTION OF ANY SUCH CERTIFICATE.

The following abbreviations, when used in the inscription on the face of this certificate, shall be construed as though they were written out in full according to applicable laws or regulations:

Additional abbreviations may also be used though not in the above list.

For value received, ___________________________________ hereby sells, assigns and transfers unto

2

Exhibit 99.1

C3IS INC. ANNOUNCES REVERSE STOCK SPLIT

To be effective January 2, 2025

Aiming to meet the minimum bid price requirement for maintaining listing on Nasdaq

ATHENS, GREECE – December 30, 2024 – C3IS INC. (the “Company”) (Nasdaq: CISS) today announced that its board of directors has determined to effect a one-for-two-and-a-half (1-for-2.5) reverse stock split of the Company’s common stock, par value $0.01 per share.

The reverse stock split will take effect at 11:59 pm Eastern Time on December 31, 2024, and the Company’s common stock will begin trading on a split-adjusted basis on The Nasdaq Capital Market (“Nasdaq”) as of the opening of trading on January 2, 2025. The CUSIP number of Y18284 151 will be assigned to the Company’s common stock when the reverse stock split becomes effective.

When the reverse stock split becomes effective, every two and one-half (2.5) of the Company’s issued shares of common stock will be combined into one issued share of common stock, without any change to the par value per share. This will reduce the number of outstanding shares of common stock from approximately 10.6 million shares to approximately 4.2 million shares. The Company’s outstanding warrants and Series A Convertible Preferred Stock will be proportionately adjusted to increase the exercise price and reduce the number of shares issuable upon exercise. With respect to the Company’s Class B Warrants and Class C Warrants, the exercise price and number of shares issuable upon exercise will be adjusted further in an adjustment period ending on the fifth trading day after the effective time of the reverse split pursuant to the terms of such warrants.

No fractional shares will be issued in connection with the reverse stock split. Stockholders who would otherwise hold a fraction of a share of common stock of the Company will receive a cash payment in lieu thereof at a price equal to that fraction of a share to which the stockholder would otherwise be entitled, multiplied by the closing price of the Company’s common stock on Nasdaq on December 31, 2024 (as adjusted for the reverse split).

Stockholders with shares held in book-entry form or through a bank, broker, or other nominee are not required to take any action and will see the consequence of the reverse stock split reflected in their accounts on or after January 2, 2025. Such beneficial holders may contact their bank, broker, or nominee for more information.

The reverse stock split ratio approved by the board of directors is within the, previously disclosed, range of ratios for a reverse stock split authorized by the stockholders of the Company, within three years of the spin-out of the Company.

The purpose of the reverse stock split is to increase the market price of the Company’s common stock. The Company believes that the reverse stock split will increase the market price for its common stock and allow it to satisfy the minimum bid price requirement for maintaining listing on Nasdaq.

Forward-Looking Statements

Matters discussed in this release may constitute forward-looking statements within the meaning of the safe harbor provisions of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934, including statements with respect to the C3is Inc.’s ability to maintain compliance with Nasdaq’s continued listing standards and remain listed on Nasdaq or other major stock exchange and other statements that are forward looking. 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 release are based upon various assumptions. 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 U.S. Securities and Exchange Commission (the “SEC”) and the following: the strength of world economies and currencies, global 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 dry-dockings, shipyard performance, changes in C3is Inc.’s operating expenses, including bunker prices, drydocking and insurance costs, ability to fund the remaining purchase price for one of our drybulk vessels, ability to obtain financing and comply with covenants in our financing arrangements, 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.

Risks and uncertainties are further described in reports filed by C3is Inc. with the SEC.

ABOUT C3IS INC.

C3is Inc. is a ship-owning company providing dry bulk and crude oil seaborne transportation services. The Company owns four vessels, three handysize drybulk carriers with a total capacity of 97,664 deadweight tons (dwt) and an Aframax oil tanker with a cargo carrying capacity of approximately 115,800 dwt, resulting with a fleet total capacity of 213,464 dwt. C3is Inc.’s shares of Common Stock are listed on the Nasdaq Capital Market and trade under the symbol “CISS.”

For further information, please contact:

Company Contact:

Nina Pyndiah

Chief Financial Officer

C3is INC.

00-30-210-6250-001

E-mail: info@c3is.pro