C3is Inc. (NASDAQ: CISS) adds $22.9M product tanker, lifting fleet to six ships

Rhea-AI Filing Summary

C3is Inc. filed a prospectus supplement to update its existing offering documents with new information about a vessel purchase. Through an attached Form 6-K, the company reports an agreement to acquire a 2011 South Korean-built product tanker with 47,203 deadweight tons capacity for $22.9 million from an entity affiliated with Brave Maritime Corp Inc. The deal was approved by the company’s independent directors, who obtained independent valuations and used the average of those values to set the price.

The vessel is expected to be delivered between Q1 and Q2 2026, and C3is has the option to pay the purchase price one year after the agreement date without interest. After this vessel is delivered, and assuming only the previously announced product tanker acquisition also closes, C3is expects to operate a fleet of six vessels with total capacity of 310,667 deadweight tons, including three Handysize drybulk carriers, one Aframax oil tanker and two product tankers.

Insights

Maritime equity analyst

C3is adds a $22.9M product tanker, expanding fleet to six vessels.

C3is Inc. has agreed to buy a 2011 South Korean-built product tanker with 47,203 deadweight tons for $22.9 million from an entity affiliated with Brave Maritime. The price was benchmarked to independent vessel valuations, with the company using the average of those values, and the transaction was approved by independent directors, which helps address potential related-party concerns.

The vessel is expected to be delivered between Q1 and Q2 2026, and C3is has the option to defer payment for one year from the agreement date without interest, which may ease near-term cash outlay. Assuming this delivery and a previously announced product tanker acquisition, the fleet is projected to reach six vessels with total capacity of 310,667 deadweight tons, shifting the mix toward tankers alongside existing Handysize drybulk and Aframax tonnage.

Filed Pursuant to Rule 424(b)(3)

Registration Nos. 333-276430 and 333-276597

PROSPECTUS SUPPLEMENT NO. 2

(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 20, 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 20, 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

Vessel Acquisition Agreement

C3is Inc. (the “Company”) has entered into an agreement to acquire a 2011 South Korean-built product tanker with 47,203 deadweight tons (dwt) capacity for a purchase price of $22.9 million, from an entity affiliated with Brave Maritime Corp Inc. The transaction with Brave Maritime, which is affiliated with members of the Vafias family, was approved by the independent directors of the Company, which obtained independent valuations of the vessel, and used the average price of these valuations.

The vessel is expected to be delivered between Q1 and Q2 2026. The Company has the option to pay for the vessel one year after the date of the purchase agreement, without interest.

Following this vessel delivery, and assuming no further acquisitions or dispositions other than the pending product tanker acquisition previously announced, the Company’s fleet will total 6 vessels, three Handysize drybulk carriers with a total capacity of 97,664 dwt, an Aframax oil tanker with a capacity of approximately 115,800 dwt and two product tankers with a capacity of approximately 97,203 dwt, resulting in a fleet total capacity of 310,667 dwt.

*****

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.

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 20, 2026

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

FAQ

What does C3is Inc. (CISS) disclose in this prospectus supplement?

The supplement updates C3is Inc.’s existing prospectus by incorporating a Form 6-K that reports an agreement to acquire a 2011 South Korean-built product tanker for $22.9 million and describes the resulting fleet profile after delivery.

What vessel is C3is Inc. (CISS) acquiring and for how much?

C3is Inc. has agreed to acquire a 2011 South Korean-built product tanker with 47,203 deadweight tons capacity for a purchase price of $22.9 million from an entity affiliated with Brave Maritime Corp Inc.

When is the new product tanker expected to be delivered to C3is Inc. (CISS)?

The company expects the newly acquired 2011 product tanker to be delivered between Q1 and Q2 2026, according to the Form 6-K incorporated into the prospectus supplement.

How will this acquisition affect the C3is Inc. (CISS) fleet size and capacity?

Assuming this vessel is delivered and the previously announced product tanker acquisition is completed, C3is Inc. expects to operate six vessels with a total capacity of 310,667 deadweight tons, including three Handysize drybulk carriers, one Aframax oil tanker and two product tankers.

How was the price for the new vessel determined by C3is Inc. (CISS)?

Independent directors of C3is Inc. obtained independent valuations of the vessel and used the average price of these valuations to approve the $22.9 million purchase from an entity affiliated with Brave Maritime Corp Inc.

What payment terms did C3is Inc. (CISS) secure for the vessel acquisition?

C3is Inc. has the option to pay the purchase price for the product tanker one year after the date of the purchase agreement, without interest, which defers cash payment while securing the vessel.

How is this Form 6-K used in C3is Inc. (CISS) registration statements?

The Form 6-K describing the vessel acquisition is incorporated by reference into C3is Inc.’s Registration Statements on Form S-8 (Reg. No. 333-273306) and Form F-3 (Reg. No. 333-285135).