Hydrogen　spillover,　involving　the　surface　migration　of　dissociated　hydrogen　atoms　from　active　metal　sites　to　the　relatively　inert　catalyst　support,　plays　a　crucial　role　in　hydrogen-involved　catalytic　processes.　However,　a　comprehensive　understanding　of　how　H　atoms　are　driven　to　spill　over　from　active　sites　onto　the　catalyst　support　is　still　lacking.　Here,　we　examine　the　atomic-scale　perspective　of　the　H　spillover　process　on　a　Pt/Cu(111)　single　atom　alloy　surface　using　machine-learning　accelerated　molecular　dynamics　calculations　based　on　density　functional　theory.　Our　results　show　that　when　an　impinging　H2　dissociates　at　an　active　Pt　site,　the　Pt　atom　undergoes　deactivation　due　to　the　dissociated　hydrogen　atoms　that　attach　to　it.　Interestingly,　collisions　between　H2　and　sticking　H　atoms　facilitate　H　spillover　onto　the　host　Cu,　leading　to　the　reactivation　of　the　Pt　atom　and　the　realization　of　a　continuous　H　spillover　process.　This　work　underscores　the　importance　of　the　interaction　between　gas　molecules　and　adsorbates　as　a　driving　force　in　elucidating　chemical　processes　under　a　gaseous　atmosphere,　which　has　so　far　been　underappreciated　in　thermodynamic　studies.　©　2023　Wiley-VCH　GmbH.