This book constitutes the refereed proceedings of the 21st Brazilian Symposium on Formal Methods, SBMF 2018, which took place in Salvador, Brazil, in November 2018. The 16 regular papers presented in this book were carefully reviewed and selected from 30 submissions. The papers are organized in topical sections such as: techniques and methodologies; specification and modeling languages; theoretical foundations; verification and validation; experience reports regarding teaching formal methods; and applications. Chapter “TeSSLa: Temporal Stream-Based Specification Language” is available open access under a Creative Commons Attribution 4.0 International License via link.springer.com.