Filed Pursuant to Rule 424(b)(3)
Registration No. 333-290011
PROSPECTUS SUPPLEMENT NO. 2
(TO PROSPECTUS DATED
DECEMBER 11, 2025)
C3IS INC.
This is a supplement (“Prospectus Supplement”) to the prospectus, dated December 11, 2025 (“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 Reports 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 14 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
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| C3IS INC. |
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| By: |
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/s/ Nina Pyndiah |
| Name: |
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Nina Pyndiah |
| Title: |
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Chief Financial Officer |
