6-K - COMPANHIA DE SANEAMENTO BASICO DO ESTADO DE SAO PAULO-SABESP (0001170858) (Filer)

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

FORM 6-K

REPORT OF FOREIGN ISSUER
PURSUANT TO RULE 13a-16 OR 15d-16 OF THE

SECURITIES EXCHANGE ACT OF 1934

For October, 2026

(Commission File No. 1-31317)

Companhia de Saneamento Básico do Estado de São Paulo - SABESP

(Exact name of registrant as specified in its charter)

Basic Sanitation Company of the State of Sao Paulo - SABESP

(Translation of Registrant's name into English)

Rua Costa Carvalho, 300
São Paulo, S.P., 05429-900
Federative Republic of Brazil

(Address of Registrant's principal executive offices)

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

Form 20-F ___X___ 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)__.

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)__.

Indicate by check mark whether the registrant by furnishing the
information contained in this Form is also thereby furnishing the
information to the Commission pursuant to Rule 12g3-2(b) under
the Securities Exchange Act of 1934.

Yes ______ No ___X___

If "Yes" is marked, indicated below the file number assigned to the
registrant in connection with Rule 12g3-2(b):

JOINT MATERIAL FACT

RESULT OF THE EXERCISE OF THE WITHDRAWAL RIGHT AND CONFIRMATION OF THE TIMETABLE OF THE MERGER OF SHARES

Companhia de Saneamento Básico do Estado de São Paulo – SABESP ("SABESP") and EMAE – Empresa Metropolitana de Águas e Energia S.A. ("EMAE" and, together with SABESP, the "Companies"), pursuant to Article 157, Paragraph 4, of Law No. 6,404, dated December 15, 1976 ("Brazilian Corporations Law") and the provisions of CVM Resolution No. 44, dated August 23, 2021, as amended, further to the Joint Material Facts disclosed on April 24 and June 29, 2026 by the Companies; the Joint Notice to the Market disclosed on July 27, 2026 by the Companies; EMAE's Material Fact dated July 29, 2026; the Material Fact disclosed by Sabesp on July 30, 2026; and the Joint Material Facts disclosed by the Companies on September 16 and October 8, 2026, in the context of the merger by SABESP of the shares issued by EMAE ("Merger of Shares"), hereby present to their shareholders and the market in general the following information.

I. End of the Period and Result of the Exercise of the Withdrawal Right.

On October 19, 2026, the period for the exercise of the withdrawal right ("Withdrawal Right") by the shareholders dissenting from the resolution taken at EMAE's Extraordinary Shareholders' Meeting held on September 16, 2026, which approved the Merger of Shares, came to an end.

Given that EMAE did not receive any request for the exercise of the Withdrawal Right, the Companies clarify that there will be no amount to be paid as reimbursement to shareholders. Accordingly, the Companies clarify that the prerogative set forth in Article 137, Paragraph 3, of the Brazilian Corporations Law is not applicable.

II. Consummation of the Merger of Shares.

Pursuant to Exhibit I to this Material Fact, October 23, 2026 will be the last trading day for shares issued by EMAE on B3 S.A. – Brasil, Bolsa, Balcão ("B3") and, on October 27, 2026, EMAE shareholders holding shares on such date will receive shares issued by SABESP, in accordance with the exchange ratio of 1.31950000000 common shares issued by SABESP for each 1 (one) common or preferred share issued by EMAE, traded under the ticker SBSP3.

III. Fractional Shares.

Any fractional common shares issued by Sabesp resulting from the Merger of Shares will be grouped into whole numbers and subsequently sold on the spot market operated by B3 after the consummation of the Merger of Shares, pursuant to a notice to be duly disclosed to the market by SABESP, as applicable.

IV. Tax Treatment in the Merger of Shares.

As informed by the Companies in the Joint Material Fact disclosed on September 16, 2026, current EMAE shareholders may be subject to the payment of income tax on any capital gain in connection with the Merger of Shares, in addition to other taxes, in accordance with the legal and regulatory rules applicable to each category of investor. Any tax impacts arising from the Merger of Shares, including Income Tax on capital gains of EMAE shareholders, whether resident or non-resident in Brazil, shall be the sole responsibility of the respective shareholders, subject to applicable tax legislation.

Shareholders Resident in Brazil. EMAE shareholders resident in Brazil shall be responsible for any payment of the taxes applicable to them, and should consult their own advisors regarding the applicable tax treatment.

Shareholders Non-Resident in Brazil. Sabesp, in its capacity as the acquirer of the shares issued by EMAE, is responsible for withholding the Withholding Income Tax ("WHT") on any capital gain calculated in connection with the Merger of Shares of investors who are non-tax residents in Brazil ("NRI"), pursuant to applicable tax legislation.

The tax base for the WHT shall be the capital gain of the respective NRI, corresponding to the positive difference, if any, between (i) the per-share value of EMAE attributed in the exchange ratio under the Merger of Shares; and (ii) the acquisition cost of the shares issued by EMAE held by each NRI. Sabesp shall use the information provided by the NRI (directly or through their custody agents) for the calculation of the capital gain, and the NRI shall be responsible for the accuracy and completeness of such information and documents.

Sabesp: (i) shall consider the acquisition cost to be zero for NRI that fail to submit the acquisition cost of the shares issued by EMAE, or that submit the information and/or supporting documentation in an inadequate, insufficient, or untimely manner; and (ii) shall apply the rate of 25% on the capital gains of NRI that, within the same deadline, fail to inform their country or dependency of tax residence or domicile, or fail to confirm their classification under CMN/CVM Joint Resolution No. 13, in accordance with applicable legislation and regulations.

Sabesp may charge or offset the amount of WHT eventually withheld on behalf of the NRI by deducting from the number of SABESP shares that would be delivered to the shareholder an amount equivalent to that due for payment of the tax.

São Paulo, October 20, 2026

EXHIBIT I

TIMETABLE OF THE MERGER OF SHARES

SIGNATURE  

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, in the city São Paulo, Brazil.

Date: October 21, 2026

FORWARD-LOOKING STATEMENTS

This press release may contain forward-looking statements. These statements are statements that are not historical facts, and are based on management's current view and estimates of future economic circumstances, industry conditions, company performance and financial results. The words "anticipates", "believes", "estimates", "expects", "plans" and similar expressions, as they relate to the company, are intended to identify forward-looking statements. Statements regarding the declaration or payment of dividends, the implementation of principal operating and financing strategies and capital expenditure plans, the direction of future operations and the factors or trends affecting financial condition, liquidity or results of operations are examples of forward-looking statements. Such statements reflect the current views of management and are subject to a number of risks and uncertainties. There is no guarantee that the expected events, trends or results will actually occur. The statements are based on many assumptions and factors, including general economic and market conditions, industry conditions, and operating factors. Any changes in such assumptions or factors could cause actual results to differ materially from current expectations.