This book constitutes the thoroughly refereed post-conference proceedings of the 12th Brazilian Symposium on Formal Methods, SBMF 2009, held in Gramado, Brazil, in August 2009 -- co-located with SAST 2009, the Brazilian Workshop on Systematic and Automated Software Testing. The 20 revised full papers presented together with 3 invited papers were carefully selected from numerous submissions during two rounds of reviewing and improvement. The papers are devoted to the dissemination of the development and use of formal methods for the design and verification of computational systems. The symposium provided an opportunity for researchers with a broad range of interests in formal methods for developing computing systems and software to discuss recent developments in this field.