CSAN 6-K

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

Report of Foreign Issuer

Pursuant To Rule 13a-16 Or 15d-16 of the

Securities Exchange Act of 1934

For the month of August 2026

Commission File Number: 333-251238

COSAN S.A.

(Exact name of registrant as specified in its charter)

N/A

(Translation of registrant’s name into English)

Av. Brigadeiro Faria Lima, 4100, – 16th floor São Paulo, SP 04538-132 Brazil (Address of principal executive offices)

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 ☐

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1):

Yes ☐ No ☒

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7):

Yes ☐ No ☒

COSAN S.A.

CNPJ nº 50.746.577/0001-15

NIRE 35.300.177.045

Publicly Held Company

CVM Code 01983-6

MATERIAL FACT

COSAN S.A. (B3: CSAN3; NYSE: CSAN) (“Cosan” or the “Company”), in compliance with Brazilian Securities and Exchange Commission (“CVM”) Resolution No. 44/2021 and further to the Notice to the Market disclosed on June 29, 2026, hereby informs its shareholders and the market in general that, in line with its deleveraging strategy, it has continued to advance the process of evaluating alternatives for the sale of a portion of its equity interest in Rumo S.A. (“Rumo”).

As reported in recent media articles, such process is in the phase of submission, by potential interested parties, of binding proposals. The Company has not made any decision, to date, regarding the execution of a transaction involving its stake in Rumo.

The Company will keep the market informed of any material developments related to this process.

São Paulo, August 20, 2026.

Rafael Bergman

Chief Financial and Investor Relations Officer

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: August 20, 2026