Brighton downed Arsenal who fell to their second successive defeat, putting the race for a top-four spot in the Premier League out of their hands.
Enock Mwepu set up Leandro Trossard for the first and then scored the second to seal the game.
Arsenal were given hope a minute from time when less than 60 seconds after hitting the bar from a free-kick – and Eddie Nketiah smashing the follow up against the woodwork – Martin Odegaard’s effort from 30 yards was deflected off Danny Welbeck and flew over the head of Robert Sanchez.
In-between Arsenal had a goal ruled out by VAR when Gabriel Martinelli headed home from a yard out.
The result leaves the Gunners, who fell to a 3-0 defeat at Crystal Palace on Monday, behind their North London rivals Tottenham.
Brighton, who snapped a run of six defeats and a draw in seven games, rise to 11th spot.