This book constitutes the refereed proceedings of the 20th International Conference on Theory and Applications of Satisfiability Testing, SAT 2017, held in Melbourne, Australia, in August/September 2017. The 22 revised full papers, 5 short papers, and 3 tool papers were carefully reviewed and selected from 64 submissions. The papers are organized in the following topical sections: algorithms, complexity, and lower bounds; clause learning and symmetry handling; maximum satisfiability and minimal correction sets; parallel SAT solving; quantified Boolean formulas; satisfiability modulo theories; and SAT encodings.