The intrinsic activities of six important transition metals for HI decomposition were identified by kinetic study. Ru showed high activity and a low apparent activation barrier, indicating a good activity at low temperature. Ru/AC (Ru clusters supported on activated carbon) was designed and proved to be a highly active, cost-effective, and long-term durable catalyst for HI decomposition. The present work could contribute to catalyst development for the industrialization of the sulfur-iodine thermochemical cycle for hydrogen production.