Jane Street reports 7.2% C3is Inc. stake

C3is Inc. (CISS) has a new significant shareholder disclosure.

Rhea-AI Filing Summary

C3is Inc. (CISS) has a new significant shareholder disclosure. Jane Street Group, LLC, together with its subsidiaries Jane Street Capital, LLC and Jane Street Global Trading, LLC, reports beneficial ownership of 937,330 shares of C3is Inc. common stock, representing 7.2% of the outstanding class. All of these shares are held with shared voting and dispositive power; none are held with sole power. Within this total, Jane Street Capital, LLC is reported to beneficially own 678,960 shares (5.2%) and Jane Street Global Trading, LLC 258,370 shares (2.0%). The filing identifies Jane Street Group, LLC as the parent holding company, with the two subsidiaries as the entities that acquired the securities.

Key Figures

Total shares beneficially owned: 937,330 shares Percent of class owned: 7.2% Shares with shared voting power: 937,330 shares +4 more

7 metrics

Total shares beneficially owned 937,330 shares Beneficial ownership by Jane Street Group, LLC and subsidiaries in C3is Inc. common stock

Percent of class owned 7.2% Percentage of C3is Inc. common stock beneficially owned by Jane Street Group, LLC

Shares with shared voting power 937,330 shares Number of shares over which Jane Street entities have shared voting power

Jane Street Capital holdings 678,960 shares C3is Inc. shares beneficially owned by Jane Street Capital, LLC (5.2% of class)

Jane Street Global Trading holdings 258,370 shares C3is Inc. shares beneficially owned by Jane Street Global Trading, LLC (2.0% of class)

Sole voting power 0 shares Shares of C3is Inc. over which Jane Street entities report sole voting power

Sole dispositive power 0 shares Shares of C3is Inc. over which Jane Street entities report sole dispositive power

Key Terms

beneficially owned, shared voting power, shared dispositive power, parent holding company

4 terms

beneficially owned financial

"Item 4. | Ownership (a) | Amount beneficially owned: 937,330.00"

Beneficially owned describes securities or assets where a person has the economic rights and control—such as the right to receive dividends and to direct voting—even if legal title is held in another name. Think of it like having the keys and using a car that’s registered to someone else: you get the benefits and make decisions. Investors care because beneficial ownership reveals who truly controls value and voting power, affecting corporate decisions and takeover dynamics.

shared voting power financial

"Shared Voting Power 937,330.00"

Shared voting power occurs when two or more parties jointly have the right to vote or decide how a block of company shares is cast, like co-owners who must agree before moving a piece of furniture. Investors care because who controls voting rights affects board elections, major corporate decisions and takeover outcomes, and shared control can alter regulatory disclosures and the practical influence any holder has over a company’s direction and value.

shared dispositive power financial

"Shared Dispositive Power 937,330.00"

parent holding company financial

"If a parent holding company has filed this schedule, pursuant to (ii)(G)..."

FAQ

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

What percentage of C3is Inc. (CISS) does Jane Street Group report owning?

Jane Street Group, LLC reports beneficial ownership of 7.2% of C3is Inc.'s common stock. This corresponds to 937,330 shares held with shared voting and dispositive power through its subsidiaries Jane Street Capital, LLC and Jane Street Global Trading, LLC.

How many C3is Inc. (CISS) shares does Jane Street Group beneficially own?

Jane Street Group, LLC reports beneficial ownership of 937,330 C3is Inc. common shares. All of these shares are reported with shared voting and shared dispositive power, and none are reported with sole voting or dispositive power by Jane Street entities.

What is Jane Street Capital, LLC’s ownership stake in C3is Inc. (CISS)?

Jane Street Capital, LLC reports beneficial ownership of 678,960 C3is Inc. shares, equal to 5.2% of the common stock. These shares are held with shared voting and shared dispositive power under the broader Jane Street Group, LLC structure.

What is Jane Street Global Trading, LLC’s ownership stake in C3is Inc. (CISS)?

Jane Street Global Trading, LLC reports beneficial ownership of 258,370 C3is Inc. shares, representing 2.0% of the common stock. These shares also carry shared voting and shared dispositive power rather than sole authority by the subsidiary.

Who are the reporting persons for this Schedule 13G related to C3is Inc. (CISS)?

The reporting persons are Jane Street Group, LLC, Jane Street Capital, LLC, and Jane Street Global Trading, LLC. Jane Street Group, LLC is identified as a parent holding company, and the two LLCs are the subsidiaries that hold the C3is Inc. shares.

Does Jane Street Group have sole or shared voting power over its C3is Inc. (CISS) shares?

Jane Street entities report 0 shares with sole voting power and 937,330 shares with shared voting power. They likewise report 0 shares with sole dispositive power and the same 937,330 shares with shared dispositive power.

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

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Available on EDGAR 08/19/2026 - 04:17 PM Accepted by SEC EDGAR 08/19/2026 - 04:15 PM Learn about SEC filing dates

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

C3is Inc.

(Name of Issuer)

common stock, par value $0.01 per share

(Title of Class of Securities)

Y18284300

(CUSIP Number)

08/13/2026

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

Rule 13d-1(b)

Rule 13d-1(c)

Rule 13d-1(d)

schemaVersion:

SCHEDULE 13G

CUSIP Number(s): Y18284300

1

Names of Reporting Persons

JANE STREET GROUP, LLC

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 937,330.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 937,330.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

937,330.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

7.2 %

12

Type of Reporting Person (See Instructions)

HC

SCHEDULE 13G

CUSIP Number(s): Y18284300

1

Names of Reporting Persons

Jane Street Capital, LLC

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 678,960.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 678,960.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

678,960.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

5.2 %

12

Type of Reporting Person (See Instructions)

BD

SCHEDULE 13G

CUSIP Number(s): Y18284300

1

Names of Reporting Persons

Jane Street Global Trading, LLC

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 258,370.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 258,370.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

258,370.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

2.0 %

12

Type of Reporting Person (See Instructions)

OO

SCHEDULE 13G

Item 1.
(a)	Name of issuer: C3is Inc.
(b)	Address of issuer's principal executive offices: 331 KIFISSIAS AVENUE, ERITHREA, ATHENS, GREECE, 14561.
Item 2.
(a)	Name of person filing: Jane Street Group, LLC; Jane Street Capital, LLC; Jane Street Global Trading, LLC
(b)	Address or principal business office or, if none, residence: Jane Street Group, LLC 250 Vesey Street 3rd Floor New York, NY 10281 Jane Street Capital, LLC 250 Vesey Street 3rd Floor New York, NY 10281 Jane Street Global Trading, LLC 250 Vesey Street 3rd Floor New York, NY 10281
(c)	Citizenship: See Item 4 of Cover Page
(d)	Title of class of securities: common stock, par value $0.01 per share
(e)	CUSIP Number(s): Y18284300
Item 3.	If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
(a)	Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)	Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)	Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)	Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)	An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)	An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)	A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)	A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)	A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)	A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:
(k)	Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.	Ownership
(a)	Amount beneficially owned: 937,330.00
(b)	Percent of class: 7.2%
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: 0
	(ii) Shared power to vote or to direct the vote: 937,330.00
	(iii) Sole power to dispose or to direct the disposition of: 0
	(iv) Shared power to dispose or to direct the disposition of: 937,330.00
Item 5.	Ownership of 5 Percent or Less of a Class.
	Not Applicable
Item 6.	Ownership of more than 5 Percent on Behalf of Another Person.
	Not Applicable
Item 7.	Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
	If a parent holding company has filed this schedule, pursuant to Rule 13d-1(b)(ii)(G), so indicate under Item 3(g) and attach an exhibit stating the identity and the Item 3 classification of the relevant subsidiary. If a parent holding company has filed this schedule pursuant to Rule 13d-1(c) or Rule 13d-1(d), attach an exhibit stating the identification of the relevant subsidiary. Subsidiary Jane Street Capital, LLC Jane Street Global Trading, LLC
Item 8.	Identification and Classification of Members of the Group.
	Not Applicable
Item 9.	Notice of Dissolution of Group.
	Not Applicable

Item 10.

Certifications:

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under § 240.14a-11.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

JANE STREET GROUP, LLC

Signature:	/s/ Jeremy Kahn
Name/Title:	Jeremy Kahn (Authorized Signatory)
Date:	08/19/2026

Jane Street Capital, LLC

Signature:	/s/ Jeremy Kahn
Name/Title:	Jeremy Kahn (Authorized Signatory)
Date:	08/19/2026

Jane Street Global Trading, LLC

Signature:	/s/ Jeremy Kahn
Name/Title:	Jeremy Kahn (Authorized Signatory)
Date:	08/19/2026