This book constitutes the proceedings of the 27th International Symposium on Stabilization, Safety, and Security of Distributed Systems, SSS 2025, held in Kathmandu, Nepal, during October 9–11, 2025.The 21 full papers and 7 short papers together with 4 invited papers included in this book were carefully reviewed and selected from 59 submissions. The conference focuses on the design and development of distributed systems with a focus on systems that are able toprovide guarantees on their correctness, performance, and/or security in the face of an adverse operational environment.