This book constitutes the refereed proceedings of the 23rd International Symposium on Stabilization, Safety, and Security of Distributed Systems, SSS 2021, held virtually, in November 2021. The 16 full papers, 10 short and 14 invited papers presented were carefully reviewed and selected from 56 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.