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 December 2025

Commission File Number: 001-40155

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

Graphics

COSAN S.A.

CNPJ~~/MF~~ n°No. 50.746.577/0001-1515NIRE 35.300.177.045Publicly Held CompanyCVM Code 19836

NIRE 35.300.177.045

Public Company

CVM 19836

MATERIAL FACT

COSAN S.A. (B3: CSAN3; NYSE: CSAN) (“Cosan” or “Company”), in compliance with CVM Resolution No. 44, dated August 23, 2021, ~~hereby~~ informs its shareholders and the market in general that, on ~~December~~this 22date, ~~2025,~~Banco itBradesco ~~carried~~BBI ~~out~~S.A. a(“Bradesco ~~transaction~~BBI”) ~~consisting~~and ~~of:~~Banco BTG Pactual S.A. (i“BTG Pactual”) ~~the~~have ~~sale~~acquired, ofin ~~common~~equal proportions, preferred shares issued by ~~its~~Cosan ~~subsidiary,~~Dez ~~Rumo~~Participações S.A. (“~~Rumo~~Cosan Dez”), ~~equivalent~~for toa ~~approximately~~total amount of R$4 billion.~~96%~~ ofThese ~~Rumo’s~~shares ~~total~~correspond, ~~share~~in ~~capital~~terms ~~(“Shares”);~~of ~~and~~equity ~~simultaneously(ii)~~value, ~~the~~to ~~execution~~approximately 23% of ~~derivative~~Cosan ~~instruments~~Dez’s (total ~~return~~share ~~swap),~~capital ~~which~~and ~~represents~~jointly ~~the~~grant ~~same~~political ~~economic~~rights ~~exposure~~equivalent ~~corresponding~~ to ~~the~~9.98% ~~Shares~~of ~~mentioned in~~ the ~~previous~~voting ~~item~~capital (~~“Financial~~the “Transaction”).

~~The~~Cosan ~~Financial~~Dez is the vehicle that holds the shares of Compass Gás e Energia S.A. (“Compass”) owned by the Company. The Transaction isrepresents ~~part~~a renegotiation of the financial terms of the structure established between the Company’s ~~strategy~~and ~~focused~~Bradesco onBBI ~~liquidity~~in 2022. Regarding the conditions agreed upon in the context of the Transaction, the Company granted Bradesco BBI and ~~cash~~BTG ~~management~~Pactual a put option on the shares of Cosan Dez held by these investors, exercisable in one or more opportunities, on a staggered basis, starting from the 5th, 6th, and ~~does~~7th ~~not~~anniversaries of the Transaction, which allowed the Company to reduce ~~Cosan~~the financial cost associated with the previous transaction. In addition, the Transaction follows the Company’s ~~political~~ongoing ~~and~~strategy ~~economic~~to ~~rights~~optimize ~~regarding~~liabilities ~~Rumo.~~following the recent capitalization.

~~Company’s~~With ~~total~~the ~~participation~~closing inof ~~Rumo’s~~the ~~total share capital remains unchanged~~Transaction, ~~compounded~~the bycorporate ~~20.33%~~structure ~~direct~~of ~~stake~~Cosan Dez and ~~9.94%~~Compass ~~via~~will ~~derivatives~~be ~~instruments.~~as follows:

~~The~~São ~~Company also informs that~~Paulo, onDecember ~~this date~~30, ~~it sent a communication to Rumo in accordance with Article 12 of CVM Resolution No. 44.~~2025

~~São~~Rafael ~~Paulo, December 22, 2025.~~Bergman

~~Rafael~~Chief ~~Bergman~~Financial and Investor Relations Officer

~~Chief Financial and Investor Relations Officer~~SIGNATURES

~~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.

~~Pursuant~~Date: December ~~to the requirements of the Securities Exchange Act of 1934~~30, ~~the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.~~ 2025

~~Date~~COSAN S.A. By: ~~December~~ ~~22,~~/s/ ~~2025~~ Rafael Bergman Name: Rafael Bergman Title: Chief Financial Officer

COSAN S.A. By: /s/ Rafael Bergman Name: Rafael Bergman Title: Chief Financial Officer

Filing Compare · Cosan S.A.