Arsene Wenger's final trip to Manchester United as Arsenal manager ended in defeat after an injury-time winner from Marouane Fellaini.
The Belgium international flicked a header past Gunners goalkeeper David Ospina after being set up by Ashley Young.
Just three minutes earlier he had headed against the post, with Marcus Rashford bundling home the rebound from an offside position.
Paul Pogba had put United in front on 16 minutes but that goal was cancelled out by Henrikh Mkhitaryan's low strike against his former club.
Source: BBC