SEC Form 6-K filed by Companhia de saneamento Basico Do Estado De Sao Paulo - Sabesp

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___

COMPANHIA DE SANEAMENTO BÁSICO DO ESTADO DE SÃO PAULO – SABESP Publicly-held company CNPJ No. 43.776.517/0001-80 NIRE No. 35.3000.1683-1 MATERIAL FACT Companhia de Saneamento Básico do Estado de São Paulo – Sabesp (the “Company”), in compliance with the provisions of Resolution No. 44, of August 23, 2021, as amended, issued by the Brazilian Securities and Exchange Commission (“CVM”), hereby informs its shareholders and the market in general that, it has entered into share purchase agreements with: (i) Vórtx Distribuidora de Títulos e Valores Mobiliários Ltda., acting as fiduciary agent on behalf of the debentureholders of the First Issuance of Simple, Non-Convertible Debentures, Secured by Fiduciary Guarantees and Personal Guarantees, in a Single Series, for Private Placement, issued by Phoenix Água e Energia S.A., for the acquisition, by the Company or through one of its subsidiaries, of 74.9% (seventy-four point nine percent) of the common shares issued by EMAE – Empresa Metropolitana de Águas e Energia S.A. (“EMAE”), at a price per share of R$59.33 (fifty-nine Reais and thirty-three cents); and (ii) Centrais Elétricas Brasileiras S.A. – Eletrobras (“Eletrobras”), for the acquisition, by the Company or through one of its subsidiaries, of 66.8% (sixty-six point eighty percent) of the preferred shares issued by EMAE, at a price per share of R$32.07 (thirty-two Reais and seven cents) The transactions were negotiated separately with the respective counterparties, and their completion is subject to approval by the competent regulatory and antitrust authorities, among other conditions precedent. Upon completion of the aforementioned transactions, the Company will hold shares representing 70.1% (seventy point one percent) of EMAE’s total share capital, at an aggregate acquisition cost of R$1,131,460,783.00 (one billion, one hundred thirty-one million, four hundred sixty thousand, seven hundred eighty-three reais). This acquisition represents a strategic milestone for the Company, bringing benefits in two complementary fronts: • Water Security: The integration of the Guarapiranga and Billings systems will allow greater flexibility in water resource management within the São Paulo Metropolitan Region, enhancing water supply security and enabling multiple uses of these water sources. • Power Assets: EMAE has a portfolio of power generation assets with solid cash flow, supported by long-term inflation-indexed revenue contracts, contributing to financial stability and sustainable value creation. By combining water security with energy potential, the acquisition enhances the synergies between Company’s business segments and strengthens the Company’s foundation to face the challenges posed by climate change and growing demand for essential services. The Company will hold a conference call with investors on Monday, October 6, at 10:00 a.m. (BRT) to provide further information regarding the transactions disclosed herein (connection details available on the Company’s Investor Relations website). The Company will keep its shareholders and the market informed of any further developments related to the subject matter of this Material Fact. São Paulo, October 5, 2025. DANIEL SZLAK CFO and IRO