This book constitutes the refereed proceedings of the 26th Brazilian Symposium on Formal Methods, SBMF 2023, held in Manaus, Brazil, during December 4-8, 2023. The 7 full papers and 2 short papers presented in this book were carefully reviewed and selected from 16 submissions. The papers are divided into the following topical sections: specification and modeling languages; testing; and verification and validation.