C3is sets 1-for-5 reverse split effective Nov. 8

The split also adjusts warrant and preferred-stock terms, while cash replaces any fractional common shares.

Available on EDGAR 11/05/2026 - 09:18 AM EST Accepted by SEC EDGAR 11/05/2026 - 09:18 AM EST Learn about SEC filing dates

Rhea-AI Filing Summary

C3is Inc. (CISS) announced a one-for-five reverse stock split of its common stock, effective at 11:59 p.m. Eastern Time on November 8, 2026; split-adjusted trading begins at the opening on November 9, 2026. Every five issued shares will be combined into one, without changing the $0.01 par value per share. The company said the split will reduce outstanding common shares from approximately 4.2 million to approximately 0.84 million. Outstanding warrants and Series A Convertible Preferred Stock will be adjusted proportionately, increasing exercise prices and reducing shares issuable upon exercise. Class B and Class C Warrants will have a further adjustment period ending on the fifth trading day after the effective time. Fractional shares will be paid in cash based on the fraction multiplied by the common stock’s November 6, 2026 closing price on Nasdaq, adjusted for the split. The company said the split is intended to increase its share price and that it believes the change will allow it to satisfy Nasdaq’s minimum bid price requirement.

Key Figures

Reverse split ratio: 1-for-5 Outstanding common shares before split: approximately 4.2 million shares Outstanding common shares after split: approximately 0.84 million shares +2 more

5 metrics

Reverse split ratio 1-for-5 C3is common stock

Outstanding common shares before split approximately 4.2 million shares Before the reverse split

Outstanding common shares after split approximately 0.84 million shares After the reverse split

Effective time 11:59 p.m. Eastern Time on November 8, 2026 Reverse split effective time

Warrant adjustment period 5 trading days Class B and Class C Warrants; ending after the effective time

Key Terms

reverse stock split, split-adjusted basis, fractional shares, Series A Convertible Preferred Stock, +1 more

5 terms

reverse stock split financial

"effect a one-for-five (1-for-5) reverse stock split"

A reverse stock split reduces a company's number of outstanding shares while raising the price per share proportionally, so the total value of each investor's holding is unchanged; a 1-for-10 split turns 100 shares worth $1 each into 10 shares worth $10 each. Companies often do this to regain compliance with an exchange's minimum price rule or to attract investors who avoid very low-priced stocks.

split-adjusted basis financial

"begin trading on a split-adjusted basis"

An adjustment to historical share prices and share counts that reflects past stock splits or reverse splits so that old data lines up with the current number of shares. Think of it like resizing an old photograph so it matches a new frame: it keeps price charts, returns and per‑share metrics comparable over time, which matters to investors who need accurate performance, valuation and trend analysis.

fractional shares financial

"No fractional shares will be issued"

Fractional shares are portions of a whole share of a stock or fund, allowing investors to own less than one full unit. They make it possible to invest a specific dollar amount rather than buy whole shares, like buying a slice of a pizza instead of the entire pie. For investors this lowers the cost barrier, helps with diversification, and lets you reinvest dividends or purchase expensive stocks in small, precise amounts.

Series A Convertible Preferred Stock financial

"outstanding warrants and Series A Convertible Preferred Stock"

Series A convertible preferred stock is a class of shares sold in an early funding round that gives investors a mix of protection and upside: it pays a priority claim over common shares if the company is sold or closes, but can be converted into ordinary shares to share in future growth. Think of it like a hybrid between a safer stake and a ticket to ownership; it matters to investors because it affects who controls the company, how future gains are split, and how much their investment is protected from downside.

adjustment period financial

"in an adjustment period ending on the fifth trading day"

Stock Split

1-for-5 reverse split · Effective November 8, 2026

Split Ratio 1-for-5 reverse split

Effective Date November 8, 2026

Shares Before Split 4,200,000

Shares After Split 840,000

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

How many CISS shares will remain after the reverse split?

C3is said the split will reduce its outstanding common shares from approximately 4.2 million to approximately 0.84 million. Every five issued shares will be combined into one.

When does the CISS reverse split take effect?

The reverse split takes effect at 11:59 p.m. Eastern Time on November 8, 2026. CISS common stock is scheduled to begin trading on a split-adjusted basis at the opening of trading on November 9, 2026.

How will the CISS reverse split affect warrants and preferred stock?

Outstanding warrants and Series A Convertible Preferred Stock will be adjusted proportionately, increasing exercise prices and reducing the number of shares issuable upon exercise. Class B and Class C Warrants will have a further adjustment period ending on the fifth trading day after the effective time.

How will CISS handle fractional shares after the reverse split?

No fractional common shares will be issued. A holder otherwise entitled to a fraction will receive cash equal to that fraction multiplied by CISS’s November 6, 2026 closing price on Nasdaq, adjusted for the reverse split.

Do CISS shareholders with book-entry shares need to take action?

Stockholders whose shares are held in book-entry form or through a bank, broker, or other nominee are not required to take action. The reverse split will be reflected in their accounts on or after November 9, 2026.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Filed Pursuant to Rule 424(b)(3)
Registration No. 333-276868

PROSPECTUS SUPPLEMENT NO. 1

(TO PROSPECTUS DATED OCTOBER 16, 2026)

C3IS INC.

This is a supplement (“Prospectus Supplement”) to the prospectus, dated October 16, 2026 (“Prospectus”), of C3is Inc. (the “Company”), which forms a part of the Company’s Registration Statement on Form F-1 (Registration No. 333-276868), 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 November 5, 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 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 November 5, 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 November 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

Attached to this report on Form 6-K as Exhibit 99.1 is a copy of C3is Inc.’s (the “Company”) press release, dated November 5, 2026, announcing a one-for-five (1-for-5) reverse stock split of the Company’s common stock, par value $0.01 per share, which will take effect at 11:59 pm Eastern Time on November 8, 2026. The Company’s common stock will begin trading on a split-adjusted basis on The Nasdaq Capital Market as of the opening of trading on November 9, 2026.

EXHIBIT INDEX

99.1 Press Release dated November 5, 2026

*****

This report on Form 6-K, including exhibit 99.1 hereto, 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.

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: November 5, 2026

C3IS INC.
By:		/s/ Nina Pyndiah
Name:		Nina Pyndiah
Title:		Chief Financial Officer

Exhibit 99.1

C3IS INC. ANNOUNCES REVERSE STOCK SPLIT

To be effective November 9, 2026

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

ATHENS, GREECE – November 5, 2026 – C3IS INC. (the “Company”) (Nasdaq: CISS) today announced that its board of directors has determined to effect a one-for-five (1-for-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 November 8, 2026, 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 November 9, 2026. The CUSIP number of Y18284508 will be assigned to the Company’s common stock when the reverse stock split becomes effective.

When the reverse stock split becomes effective, every five (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 4.2 million shares to approximately 0.84 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 November 6, 2026 (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 November 9, 2026. 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 range of ratios for a reverse stock split authorized by the stockholders 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”). Risks and uncertainties are further described in reports filed by C3is Inc. with the SEC.

ABOUT C3IS INC.

C3is Inc. is a debt-free ship-owning company providing dry bulk and crude oil seaborne transportation services. The Company owns six vessels, comprising three Handysize dry bulk carriers with a total capacity of 97,664 deadweight tons (dwt), an Aframax oil tanker with a cargo carrying capacity of 115,804 dwt and two product tankers with a total cargo carrying capacity of 97,963 dwt, resulting in a fleet total capacity of 311,431 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