Physical　exercise　is　widely　recognized　for　its　benefits　to　individuals’　general　health,　yet　its　implications　for　in-role　and　extrarole　job　performance,　especially　on　demanding　workdays,　have　rarely　been　explored.　This　oversight　is　concerning　as　high　work　demands　can　deter　employees　from　exercising　when　they　are　unaware　that　exercise　can　improve　their　job　performance　on　demanding　workdays.　In　this　research,　we　draw　on　the　effort–recovery　model　to　propose　that　daily　physical　exercise　not　only　promotes　next-day　well-being　but　also　enhances　next-day　in-role　job　performance　and　extrarole　organizational　citizenship　behavior　(OCB)　by　fostering　positive　affect　and　work　engagement　the　following　day.　Moreover,　these　benefits　of　daily　physical　exercise　are　more　pronounced　on　days　with　high　rather　than　low　work　demands.　Results　from　two　experience　sampling　studies　generally　support　our　hypotheses,　revealing　that　daily　physical　exercise　contributes　to　next-day　well-being,　both　self-　and　leader-rated　in-role　job　performance　and　self-rated,　but　not　leader-rated,　extrarole　OCB,　through　the　sequential　mediation　of　next-morning　positive　affect　and　next-day　work　engagement.　Furthermore,　these　benefits　of　physical　exercise　are　more　evident　on　days　when　employees　face　high　overall　work　demands　(Study　1)　and　in　particular　on　days　with　high-hindrance　demands　but　on　days　with　low-challenge　demands　(Study　2).　©　2024　American　Psychological　Association