This book constitutes the refereed proceedings of the 20th International Symposium on Stabilization, Safety, and Security of Distributed Systems, SSS 2018, held in Tokyo, Japan, in November 2018. The 24 revised full papers presented were carefully reviewed and selected from 55 submissions. The papers are organized into three tracks reflecting major trends related to distributed systems: theoretical and practical aspects of stabilizing systems; distributed networks and concurrency; and safety in malicious environments.