SBS 6-K

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 April, 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):

COMPANHIA DE SANEAMENTO BÁSICO DO ESTADODE SÃO PAULO – SABESPPUBLICLY HELD COMPANY

NIRE 35.300.016.831

CNPJ nº 43.776.517/0001-80

MATERIAL FACT

Companhia de Saneamento Básico doEstado de São Paulo S.A. (“Sabesp” or “Company”) in compliance with the terms set forth in Resolution No. 44, of August 23, 2021, as amended, of the Brazilian Securities and Exchange Commission (“CVM”), further to the material fact disclosed by the Company on March 27, 2026, hereby informs its shareholders and the market in general of the following:

At the Extraordinary Shareholders’ Meeting to be held on April 28, 2026, the Company will submit for approval the stock split of all common shares at a ratio of 1:5 by means of which each 1 (one) common share will represent 5 (five) common shares, with no change to the amount of the Company’s share capital. If approved, the following steps and dates will be relevant in relation to the Company’s common shares and American Depositary Receipts (“ADRs”).

1.    As for the common shares trading on the Brazilian Stock Exchange (B3 S.A. – Brasil, Bolsa, Balcão, or “B3”), the custodian institution, Itaú Unibanco S.A., shall take all measures to automatically distribute the new common shares to the Company’s shareholders, according to the following schedule:

2.    As for the ADRs trading in the New York Stock Exchange (“NYSE”), the depositary institution, The Bank of New York Mellon (“BNY”), shall issue new ADRs and distribute them to the holders of the ADR, as per the schedule applicable in the United States of America:

The stock split will not result in any change in the proportional ownership interest of the shareholders in the Company’s share capital. The resulting common shares and ADRs will grant their respective holders the same rights attributed to the currently existing common shares and ADRs, including with respect to the distribution of dividends and/or interest on equity and other distributions that may be made by the Company.

The Company will keep its shareholders and the market duly informed of the developments of this matter, in accordance with applicable regulations.

São Paulo, April 27, 2026

DANIEL SZLAK

Chief Financial Officer and Investors Relations Officer

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: April 27, 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.