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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM

~~FORM~~ 6-K

REPORT OF FOREIGN PRIVATE ISSUER

PURSUANT TO RULE

~~PURSUANT TO RULE~~ 13a-16 ~~OR 15d-16~~

15d-16

UNDER THE SECURITIES EXCHANGE ACT OF 1934

For the month of September 2025

Commission File Number

~~Commission File Number~~ 001-41717

C3IS INC.

(Translation of registrant’s name into English)

331 Kifissias Avenue Erithrea 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

~~Indicate by check mark whether the registrant files or will file annual reports under cover of Form~~ 20-F ~~or Form 40-F.~~

or Form

~~Form 20-F ☒ Form~~ 40-F ☐ .

Form

~~INFORMATION CONTAINED IN THIS FORM 6~~20-~~K REPORT~~ F ☒ Form

~~Financial and Operating Results for the three and six months ended June 30, 2025~~ 40-F ☐

Attached to this report on Form 6-K as Exhibit 99.1 is a copy of C3is Inc.’s (the “Company”) press release, dated September 2, 2025, announcing its preliminary financial and operating results for the three and six months ended June 30, 2025.

EXHIBIT INDEX

99.1 Press Release dated September 2, 2025


99.1		Management’s Discussion and Analysis of Financial Condition and Results of Operations and Consolidated Financial Statements for the Six Months Ended June 30, 2025

101.INS		Inline XBRL Instance Document

101.SCH		Inline XBRL Taxonomy Extension Schema

101.CAL		Inline XBRL Taxonomy Extension Calculation Linkbase

101.DEF		Inline XBRL Taxonomy Extension Definition Linkbase

101.LAB		Inline XBRL Taxonomy Extension Label Linkbase

101.PRE		Inline XBRL Taxonomy Extension Presentation Linkbase

104		Cover Page Interactive Data File (formatted as Inline XBRL and contained in Exhibit 101).

*****

~~Neither~~This ~~C3is~~report ~~Inc.’s independent accountants~~on Form 6-K, including exhibit ~~nor~~99.1 ~~any other independent accountants~~hereto, ~~have~~is ~~compiled,~~hereby ~~examined,~~incorporated orby ~~performed~~reference ~~any~~into ~~procedures with respect to~~ the ~~preliminary~~Company’s ~~financial~~Registration ~~results~~Statement ~~for~~ on Form S-8 (Reg. No. 333-273306) filed with the ~~three~~Securities and ~~six~~Exchange ~~months~~Commission ~~ended~~on ~~June~~July 3018, ~~2025~~2023 and ~~2024,~~Registration ~~nor~~Statement ~~have~~on ~~they~~Form F-3 (Reg. ~~expressed~~No. 333- ~~any~~285135) ~~opinion~~filed orwith ~~any~~the ~~other~~Securities ~~form~~and ofExchange ~~assurance~~Commission on ~~such information~~February 21, ~~and assume no responsibility for, and disclaim any association with, the preliminary financial results~~2025.

This report on Form 6-K, including exhibit 99.1 hereto other than the section entitled “CEO Dr. Diamantis Andriotis Commented:”, 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: September 229, 2025


C3IS INC.

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

Filing Compare · C3is Inc.