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 April 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 April 23, 2026, announcing a one-for-seven (1-for-7) 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 April 26, 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 April 27, 2026.

EXHIBIT INDEX


99.1		Press Release dated April 23, 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: April 23, 2026


C3IS INC.

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

C3is Inc. 6-K