This book constitutes the proceedings of the 15th International Conference on Security, Privacy, and Applied Cryptography Engineering, SPACE 2025, held in Guwahati, India, during December 16–19, 2025.The 22 full papers included in this book were carefully reviewed and selected from 103 submissions.They focus on various aspects of security, privacy, applied cryptography, and cryptographic engineering offering valuable insights and innovative solutions toward a better, more secure digital world.