<p>Hydrogen spillover involves the migration of hydrogen atoms from metal to another component, yet it remains challenging to achieve on non-reducible supports. This paper describes the observation of hydrogen spillover on non-reducible Al₂O₃-supported Pd single-atom catalysts, facilitated by phosphorus−hydroxyl (P−OH) bridge sites, for acetylene semi-hydrogenation. Through in situ spectroscopy, kinetic analysis, and computational analyses, we show that Pd single atoms promote the heterolytic activation of H₂, while phosphorus doping on Al₂O₃ lowers energy barrier for hydrogen spillover via the formation of P−OH species. Gas-phase acetylene collides and hydrogenates with as-formed P−OH, following Eley-Rideal type mechanism, making inert Al₂O₃ active in hydrogenation reaction. This unconventional relay pathway reduced Pd usage by one order of magnitude and improved catalytic stability, with over 95% ethylene selectivity at 99% acetylene conversion in ethylene-rich stream at 100 °C for more than 200 hours. This work elucidates the mechanism of hydrogen spillover on non-reducible Al₂O₃ and relay hydrogenation on supported single-atom catalysts.</p>