This book constitutes the proceedings of the 12th International Symposium on NASA Formal Methods, NFM 2020, held in Moffett Field, CA, USA, in May 2020.* The 20 full and 5 short papers presented in this volume were carefully reviewed and selected from 62 submissions. The papers are organized in the following topical sections: learning and formal synthesis; formal methods for DNNs; high assurance systems; requirement specification and testing; validation and solvers; solvers and program analysis; verification and times systems; autonomy and other applications; and hybrid and cyber-physical systems. *The conference was held virtually due to the COVID-19 pandemic. The chapter "Verifying a Solver for Linear Mixed Integer Arithmetic in Isabelle/HOL" is available open access under a Creative Commons Attribution 4.0 International License via link.springer.com.