C3is Inc. (CISS) completes 1-for-20 reverse split and adjusts warrants

Rhea-AI Filing Summary

C3is Inc. filed a prospectus supplement that incorporates a new Form 6-K describing a recently completed reverse stock split. Effective as of 11:59 p.m. Eastern time on January 25, 2026, the company implemented a one-for-20 reverse stock split of its common stock. This reduced the number of outstanding common shares from approximately 24.7 million to approximately 1.23 million, while the par value and other terms of the common stock remained unchanged.

No fractional shares were issued; holders who would otherwise receive a fraction received cash instead. Outstanding warrants and 5.00% Series A Convertible Preferred Stock were proportionately adjusted for the split, and certain warrant exercise and preferred conversion prices will be further adjusted based on the lowest daily volume weighted average price over a specified 10‑trading‑day window around the effective date. The post-split common shares trade on the Nasdaq Capital Market under the symbol CISS with a new CUSIP number Y18284 177.

Insights

Equity Capital Structure Analyst

C3is consolidates shares 1‑for‑20 and adjusts related securities.

C3is Inc. has executed a one-for-20 reverse stock split effective as of January 25, 2026, cutting outstanding common shares from approximately 24.7 million to approximately 1.23 million. Each block of 20 pre-split shares now represents one post-split share, while the par value of $0.01 per share and other terms of the common stock are unchanged. Cash is paid in lieu of any fractional shares, so every holder’s position is converted into whole shares plus, if needed, a small cash amount.

The filing also describes mechanical adjustments to derivative securities. Outstanding warrants and the 5.00% Series A Cumulative Convertible Perpetual Preferred Stock are proportionately adjusted so that exercise and conversion economics remain aligned with the new share count. For the Class B-1, B-2, C-1 and C-2 Warrants, and related preferred stock, the exercise and conversion prices will be further reset to the lowest daily volume weighted average price during the five trading days before and after the effective time if that price is lower, while keeping the aggregate exercise price per warrant series from their original issuance date unchanged. Some of these warrants can also be exercised through an alternative zero cash exercise exchange option.

The company’s common stock continues trading on the Nasdaq Capital Market under the symbol CISS, now with CUSIP Y18284 177, and the amendment to the articles of incorporation confirms that the total authorized common and preferred share counts and par values are not changed by this action. Overall, the disclosure is primarily structural, detailing how the reverse split and associated adjustments are implemented; the actual impact will depend on future trading prices and any exercise or conversion activity under the adjusted securities.

Filed Pursuant to Rule 424(b)(3)

Registration Nos. 333-276430 and 333-276597

PROSPECTUS SUPPLEMENT NO. 4

(TO PROSPECTUS DATED DECEMBER 18, 2025)

C3IS INC.

This is a supplement (“Prospectus Supplement”) to the prospectus, dated December 18, 2025 (“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 26, 2026 (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 8 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 26, 2026.

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 2026

Commission File Number 001-41717

C3IS INC.

(Translation of registrant’s name into English)

331 Kifissias Avenue Kifissia 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 January 23, 2026, 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 January 25, 2026, the Company effected a one-for-20 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 26, 2026. The reverse stock split reduced the number of outstanding shares of the Company’s common stock from approximately 24.7 million to approximately 1.23 million and affected all outstanding shares of common stock. No fractional shares were issued in connection with 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 zero cash 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 177, 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.

*****

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 and Registration Statement on Form F-3 (Reg. No. 333- 285135) filed with the Securities and Exchange Commission on February 21, 2025.

EXHIBIT INDEX

4.1		Articles of Amendment to Restated Articles of Incorporation.
4.2		Form of Common Stock Certificate.

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 26, 2026

C3IS INC.

By:		/s/ Nina Pyndiah
Name:		Nina Pyndiah
Title:		Chief Financial Officer

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, December 31, 2024, and April 3, 2025 (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:

(g) 2026 Reverse Stock Split. As of 11:59 p.m. Eastern time on January 25, 2026 (the “2026 Reverse Stock Split Effective Date”), each twenty (20) shares of Common Stock issued and outstanding immediately prior to the 2026 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 “2026 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 2026 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 2026 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 2026 Reverse Stock Split Effective Date, as adjusted for the reverse stock split as appropriate. Each certificate, if any, that immediately prior to the 2026 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 23rd day of January, 2026, 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.

C3IS INC.
By:		/s/ Diamantis Andriotis
		Name: Diamantis Andriotis
		Title:  Chief Executive Officer

Exhibit 4.2

COMMON STOCK

CUSIP No. Y18284 177

Certificate Number Shares

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]

Diamantis Andriotis, Chief Executive Officer, President and Director Nina Pyndiah, Secretary

COUNTERSIGNED AND REGISTERED

EQUINITI TRUST COMPANY, LLC

TRANSFER AGENT AND REGISTRAR

By

  Authorized Signature

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:

TEN COM

—

as tenants in common

UNIF GIFT MIN ACT —

Custodian

TEN ENT

—

as tenants by the entireties

(Cust)

(Minor)

JT TEN

—

as joint tenants with right

Under Uniform Gifts to Minors Act

of survivorship and not as

tenants in common

(State)

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

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

PLEASE INSERT SOCIAL SECURITY OR OTHER IDENTIFYING NUMBER OF ASSIGNEE

(PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

    Shares

represented by the within Certificate, and does hereby irrevocably constitute and appoint

    Attorney

to transfer the said shares on the books of the within named Corporation with full power of substitution in the premises.

Dated
		NOTICE:		THE SIGNATURE TO THIS ASSIGNMENT MUST CORRESPOND WITH THE NAME AS WRITTEN UPON THE FACE OF THE CERTIFICATE IN EVERY PARTICULAR, WITHOUT ALTERATION OR ENLARGEMENT OR ANY CHANGE WHATEVER.

SIGNATURE(S) GUARANTEED:

THE SIGNATURE(S) SHOULD BE GUARANTEED BY AN ELIGIBLE GUARANTOR INSTITUTION (BANKS, STOCKBROKERS, SAVINGS AND LOAN ASSOCIATIONS AND CREDIT UNIONS WITH MEMBERSHIP IN AN APPROVED SIGNATURE GUARANTEE MEDALLION PROGRAM), PURSUANT TO S.E.C. RULE 17Ad-15.

2

FAQ

What capital change did C3IS Inc. (CISS) implement in January 2026?

C3is Inc. implemented a one-for-20 reverse stock split of its common stock effective as of 11:59 p.m. Eastern time on January 25, 2026. Each 20 shares of pre-split common stock were combined into one share, consolidating the number of shares outstanding.

How did the C3IS Inc. reverse stock split affect the number of shares outstanding?

The reverse stock split reduced the number of outstanding common shares from approximately 24.7 million to approximately 1.23 million. The change applies to all outstanding common shares.

How were fractional shares handled in the C3IS Inc. (CISS) reverse stock split?

No fractional shares were issued in the reverse split. Stockholders who would otherwise be entitled to a fractional share received a cash payment in lieu of that fraction, based on the closing price per share on the Nasdaq Stock Market on the trading day immediately preceding the effective date, adjusted for the split.

Did the C3IS Inc. reverse stock split change the par value or authorized share count?

The company states that the reverse stock split did not change the par value of the common stock, which remains $0.01 per share, and did not change the number of authorized common or preferred shares or the par value of the preferred stock.

What happened to C3IS Inc. warrants and Series A Convertible Preferred Stock after the reverse split?

The company’s outstanding warrants and 5.00% Series A Cumulative Convertible Perpetual Preferred Stock were proportionately adjusted to increase exercise or conversion prices and reduce the number of shares issuable, reflecting the reverse split. For the Class B-1, B-2, C-1 and C-2 Warrants and related preferred stock, prices may be further adjusted to the lowest daily volume weighted average price over the specified 10-trading-day window around the effective time, while keeping each series’ aggregate exercise price from original issuance unchanged.

Under what symbol and CUSIP do C3IS Inc. post-split shares trade?

After the reverse stock split, C3is Inc.’s common stock continues to trade on the Nasdaq Capital Market under the symbol CISS, now with a new CUSIP number Y18284 177.

How is this reverse stock split disclosure linked to C3IS Inc. registration statements?

The Form 6-K describing the reverse stock split is attached to a prospectus supplement and is incorporated by reference into C3is Inc.’s Form S-8 (Reg. No. 333-273306) and Form F-3 (Reg. No. 333-285135), updating those registration statements with the new capital structure information.