This book constitutes the refereed proceedings of the 20th Brazilian Symposium on Formal Methods, SBMF 2017, which took place in Recifel, Brazil, in November/December 2017.The 16 papers presented together with three invited talks were carefully reviewed and selected from 37 submissions. They are organized in the following topical sections: formal methods integration and experience reports; model checking; refinement and verification; and semantics and languages. The chapter 'Rapidly Adjustable Non-Intrusive Online Monitoring for Multi-core Systems' is published open access under a CC BY 4.0 license.