This book constitutes the refereed proceedings of the Third International Conference on Cyber Warfare, Security and Space Computing, SpacSec 2025, held in Taganrog, Russia, during September 26–28, 2025.

The 18 full papers and 8 short papers included in this book were carefully reviewed and selected from 85 submissions. They focus on the multifaceted aspects of cyber warfare and space computing, including insights into the latest research and innovations.