Arsenal missed the chance to close the gap on Premier League leaders Liverpool as West Ham ended a four-match winless run with a stunning victory at Emirates Stadium.
Jarrod Bowen got the winner against a Gunners side who later had Myles Lewis-Skelly sent off.
Victory would have moved Arsenal to within five points of Arne Slot's team, who visit champions Manchester City on Sunday, but they struggled badly against a Hammers side who defended magnificently and created the better openings over the 90 minutes.
Bowen started and finished the move that led to the winner late in the first half, collecting a loose ball on the edge of his own box and feeding Aaron Wan-Bissaka before stooping low to nod the full-back's cross into the net for his 50th Premier League goal.
