This book constitutes the proceedings of the 21st International Symposium on Applied Reconfigurable Computing. Architectures, Tools, and Applications, ARC 2025, held in Seville, Spain, during April 9-11, 2025. The 12 full papers presented in this book together with 1 short paper from the technical program were carefully reviewed and selected from 40 submissions. ARC 2025 covers a wide range of topics, including hardware acceleration, security and fault tolerance, energy-efficient architectures, and emerging applications in artificial intelligence and high-performance computing. The symposium fostered collaboration and pushed the boundaries of state-of-the-art research.