This book constitutes the refereed proceedings of the 20th International Symposium on Fundamentals of Computation Theory, FCT 2015, held in Gdańsk, Poland, in August 2015. The 27 revised full papers presented were carefully reviewed and selected from 60 submissions. The papers cover topics in three main areas: algorithms, formal methods, and emerging fields and are organized in topical sections on geometry, combinatorics, text algorithms; complexity and Boolean functions; languages; set algorithms, covering, and traversal; graph algorithms and networking applications; anonymity and indistinguishability; graphs, automata, and dynamics; and logic and games.