This book constitutes the refereed proceedings of the 21st International Symposium on Stabilization, Safety, and Security of Distributed Systems, SSS 2019, held in Pisa, Italy, in October 2019.The 21 full papers presented were carefully reviewed and selected from 45 submissions. The papers deal with the design and development of distributed systems with a focus on systems that are able to provide guarantees on their structure, performance, and/or security in the face of an adverse operational environment.