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~~January ~~2025~~2026

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~~Publicly ~~No. 50.746.577/0001-15NIRE 35.300.177.045Publicly~~ Held ~~CompanyCVM Code 19836~~Company

~~MATERIAL~~CNPJ/ME ~~FACT~~No. 50.746.577/0001‑15

~~COSAN~~NIRE S35.A300.177.045 ~~(B3:~~| ~~CSAN3; NYSE: CSAN) (“Cosan” or “Company”), in compliance with~~ CVM ~~Resolution~~Code No. 44, dated August 23, 2021, informs its shareholders and the market in general that, on this date, Banco Bradesco BBI S.A. (“Bradesco BBI”) and Banco BTG Pactual S.A. (“BTG Pactual”) have acquired, in equal proportions, preferred shares issued by Cosan Dez Participações S.A. (“Cosan Dez”) for a total amount of R$4 billion. These shares correspond, in terms of equity value, to approximately 23% of Cosan Dez’s total share capital and jointly grant political rights equivalent to 9.98% of the voting capital (the “Transaction”).01983‑6

~~Cosan~~ANNOUNCEMENT ~~Dez~~REGARDING: ~~is the vehicle that holds the shares of Compass Gás e Energia S.A.~~ (~~“Compass”~~I) ~~owned~~THE byFULL ~~the~~EARLY ~~Company.~~REDEMPTION ~~The~~OF ~~Transaction~~THE ~~represents~~1ST aSERIES ~~renegotiation~~OF ofTHE ~~the~~4TH ~~financial~~ISSUANCE ~~terms~~OF ofSIMPLE, ~~the~~NON‑CONVERTIBLE, ~~structure~~UNSECURED ~~established between the Company and Bradesco BBI in 2022. Regarding the conditions agreed upon in the context of the Transaction~~DEBENTURES, ~~the~~IN ~~Company~~THREE ~~granted~~SERIES; ~~Bradesco~~(II) ~~BBI~~THE ~~and~~FULL ~~BTG~~EARLY ~~Pactual~~REDEMPTION aOF ~~put~~THE ~~option~~6TH onISSUANCE ~~the~~OF ~~shares of Cosan Dez held by these investors~~SIMPLE, ~~exercisable in one or more opportunities~~NON‑CONVERTIBLE, onUNSECURED ~~a staggered basis~~DEBENTURES, ~~starting~~IN ~~from~~A ~~the~~SINGLE ~~5th~~SERIES, ~~6th,~~BOTH ~~and~~ISSUED ~~7th~~BY ~~anniversaries~~COSAN ~~of the Transaction, which allowed the Company to reduce the financial cost associated with the previous transaction~~S.A.; InAND ~~addition,~~(III) ~~the~~THE ~~Transaction~~EXERCISE ~~follows~~OF ~~the~~THE ~~Company’s~~EARLY ~~ongoing~~CALL ~~strategy~~OPTION toRELATED ~~optimize~~TO ~~liabilities~~THE ~~following~~2029 ~~the~~SENIOR ~~recent capitalization~~NOTES.

~~With~~COSAN S.A. (B3: CSAN3; NYSE: CSAN), a corporation registered as a publicly held company with the ~~closing~~Brazilian ofSecurities and Exchange Commission (Comissão de Valores Mobiliários) in Category “A” under No. 01983‑6, headquartered in the ~~Transaction~~City of São Paulo, State of São Paulo, at Avenida Brigadeiro Faria Lima, No. 4,100, 16th floor, Suite 01, Itaim Bibi, ZIP Code 04538‑132, enrolled with the ~~corporate~~National ~~structure~~Registry of ~~Cosan~~Legal ~~Dez~~Entities of the Ministry of Finance (“CNPJ”) under No. 50.746.577/0001‑15 and ~~Compass~~registered ~~will~~with bethe asSão ~~follows~~Paulo Board of Trade under NIRE No. 35.300.177.045 (“Cosan” or “Company”), hereby informs the holders of the debentures of the:

São Paulo, December 30, 2025

(i)	the 1st (first) series of the 4th (fourth) issuance of simple, non‑convertible, unsecured debentures, in 2 (two) series, for public distribution with restricted efforts by Cosan S.A. (Code: CSAN14) (“Debentures of the 1st Series of the 4th Issuance” and “1st Series of the 4th Issuance”), issued pursuant to the “Private Instrument of the Deed of the 4th (Fourth) Issuance of Simple, Non‑Convertible, Unsecured Debentures, in 2 (Two) Series, for Public Distribution with Restricted Efforts of Cosan S.A.” executed on April 12, 2022, as amended (“Deed of the 4th Issuance”); and

(ii)	the 6th (sixth) issuance of simple, non‑convertible, unsecured debentures, in a single series, pursuant to the automatic distribution registration procedure of Cosan S.A. (Code: CSAN16) (“Debentures of the 6th Issuance” and “6th Issuance” and, together with the Debentures of the 1st Series of the 4th Issuance and the 4th Issuance, “Optional Early Redemption Debentures” and “Optional Early Redemption Issuances”), issued pursuant to the “Private Instrument of the Deed of the 6th (Sixth) Issuance of Simple, Non‑Convertible, Unsecured Debentures, in a Single Series, for Public Distribution, under the Automatic Distribution Registration Procedure, of Cosan S.A.”, executed on June 16, 2023 (“Deed of the 6th Issuance” and, together with the Deed of the 4th Issuance, the “Optional Early Redemption Deeds”).

~~Rafael~~that ~~Bergman~~will carry out the Optional Full Early Redemption of the Debentures of: (a) the 1st Series of the 4th Issuance, pursuant to Clause 5.1 of the Deed of the 4th Issuance (“Optional Early Redemption of the Debentures of the 1st Series of the 4th Issuance”); and (b) the 6th Issuance, pursuant to Clause 8.1 of the Deed of the 6th Issuance (“Optional Early Redemption of the Debentures of the 6th Issuance” and, together with the Optional Early Redemption of the Debentures of the 1st Series of the 4th Issuance, the “Optional Early Redemptions”)

~~Chief~~Terms ~~Financial~~capitalized herein and ~~Investor~~not ~~Relations~~otherwise ~~Officer~~defined in this Notice are used with the meanings ascribed to them in the Optional Early Redemption Deeds.

The Optional Early Redemption of the Debentures of the 1st Series of the 4th Issuance will take place on February 2, 2026 (“Optional Early Redemption Date of the Debentures of the 1st Series of the 4th Issuance”). On the Optional Early Redemption Date of the Debentures of the 1st Series of the 4th Issuance, the Debenture Holders will be entitled to receive the Unit Nominal Value (as defined in the Deed of the 4th Issuance) or the outstanding balance of the Unit Nominal Value of the Debentures of the First Series, as applicable, plus: (i) the Remuneration of the Debentures of the First Series (as defined in the Deed of the 4th Issuance), calculated on a pro rata temporis basis from the last remuneration payment date of the Debentures of the 1st Series of the 4th Issuance, on November 5, 2025, until the Optional Early Redemption Date of the Debentures of the 1st Series of the 4th Issuance, on February 2, 2026; and (ii) a premium equivalent to 0.35% (thirty‑five hundredths percent) per year, for the Debentures of the 1st Series of the 4th Issuance, calculated on a pro rata temporis basis, on a 252 (two hundred and fifty‑two) Business Day basis (as defined in the Deed of the 4th Issuance), considering the number of Business Days between the Optional Early Redemption Date of the Debentures of the 1st Series of the 4th Issuance (inclusive) and the Maturity Date of the Debentures of the First Series (exclusive), calculated in accordance with the formula set forth in Clause 5.1.3.(I) of the Deed of the 4th Issuance (“Optional Early Redemption Amount of the 1st Series of the 4th Issuance”).

The Optional Early Redemption of the Debentures of the 6th Issuance will take place on February 2, 2026 (“Optional Early Redemption Date of the Debentures of the 6th Issuance”). On the Optional Early Redemption Date of the Debentures of the 6th Issuance, the Debenture Holders will be entitled to receive the Unit Nominal Value (as defined in the Deed of the 6th Issuance) or the outstanding balance of the Unit Nominal Value of the Debentures of the 6th Issuance, as applicable, plus: (i) the Remuneration (as defined in the Deed of the 6th Issuance), calculated on a pro rata temporis basis from the last remuneration payment date of the Debentures of the 6th Issuance, on December 20, 2025, until the Optional Early Redemption Date of the Debentures of the 6th Issuance, on February 2, 2026; and (ii) a premium equivalent to 0.35% (thirty-five hundredths percent) per year, calculated on a pro rata temporis basis, on a 252 (two hundred and fifty‑two) Business Day basis (as defined in the Deed of the 6th Issuance), considering the number of Business Days between the Optional Early Redemption Date of the Debentures of the 6th Issuance (inclusive) and the Maturity Date (as defined in the Deed of the 6th Issuance) (exclusive), calculated pursuant to the formula set forth in Clause 8.1.3 of the Deed of the 6th Issuance (“Optional Early Redemption Amount of the 6th Issuance”).

The Company also announces that its wholly owned subsidiary, Cosan Luxembourg S.A., has decided to exercise the early call clause related to the Bonds maturing in September 2029. The early redemption of the full principal amount of US$ 504,228,000.00 will be exercised on February 17, 2026.

The Company continues to carry out its liability management process as mentioned in the offering documents of the public offerings conducted in 2025, with a focus on reducing indebtedness and improving its capital structure, as well as restoring its financial flexibility. Cosan will also keep its shareholders and the market in general informed about the next steps of this process.

São Paulo, January 16, 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: ~~December~~January 3016, ~~2025~~ 2026

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

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