This book constitutes the refereed proceedings of the 20th International Conference on Reachability Problems, RP 2026, held in Turku, Finland, during October 21–23, 2026.

The 10 full papers included in this book were carefully reviewed and selected from 21 submissions. The proceedings also contain two invited talks in full paper length. 

The papers cover topics of a wide range of disciplines and backgrounds who share an interest in reachability problems in areas such as algebraic structures, automata theory and formal languages, computational game theory, concurrency and distributed computing, decision procedures for computational models, hybrid dynamical systems, logic and model checking, and the verification of finite-and infinite-state systems.