The proceedings of the 20th Siam Physics Congress (SPC2025), held from 11–13 June 2025 at Pullman Raja Orchid, Khon Kaen, Thailand, showcase a diverse range of research across multiple areas of physics. This comprehensive compilation includes peer-reviewed contributions spanning high-energy physics, astrophysics, nanotechnology, materials science, quantum technology, physics education, and more. We invite all participants, researchers, and enthusiasts to explore the full volume of proceedings to gain insights into current scientific advancements and collaborative efforts in the Thai and international physics communities.