Neal Maupay struck late to complete a miraculous turnaround for Brighton and condemn Arsenal to their second defeat in four days.
Nicolas Pépé put Arsenal ahead with a superb curling effort before Lewis Dunk poked home the equaliser. Both sides had chances to win the game but with seconds left on the clock Maupay, who was involved in a collision with Bernd Leno that saw the goalkeeper stretchered off, lifted the ball over Emi Martínez to seal the win for Brighton.
Having played on Wednesday night, Mikel Arteta made five changes, two of which were enforced. Injuries picked up by Granit Xhaka and Pablo Marí saw Dani Ceballos and Rob Holding come into the side. Alexandre Lacazette and Pépé bolstered the attack while Sead Kolašinac replaced Kieran Tierney on his 27th birthday at left back. Mesut Özil returned to the bench.
Lining up in a 4-3-3 formation with Bukayo Saka and Ceballos playing slightly further ahead in midfield, the Gunners started well with Mattéo Guendouzi finding Pépé with a superb inside pass but his cross was slightly over-hit.
Saka looked back to his best playing on his favoured left-hand side and he almost opened the scoring with a curling effort from the edge of the box but he was denied by the crossbar.
The England youngster was the brightest creative outlet for Arsenal and he turned creator for Lacazette, who had earlier been denied by a last-ditch Dunk block, with a pinpoint cross, however, Matty Ryan was equal to the Frenchman’s header.
Most of the game was played away from Arsenal’s defensive third but a rare attack saw Maupay challenge Leno in the air and the German goalkeeper landed awkwardly. His screams said it all and after lengthy treatment he left the field on a stretcher – but made sure he made his feelings known to Maupay.
Emi Martínez replaced Leno and was called into action almost immediately with a smart low save to his right. That may have given Brighton the lift they needed but Arsenal took back control and piled on the pressure before half time with Pierre-Emerick Aubameyang only denied by a superb tackle from Adam Webster.
They carried on where they left off after the break and Aubameyang had the ball in the back of the net after Saka sent him racing through but it was wiped off for offside following a VAR check.
At the other end, Martínez did well to block Maupay from close range after a bombardment of aerial balls.
The game became end-to-end with both teams sensing a goal and the deadlock was finally broken when Pépé cut in from the right and curled an unstoppable shot beyond Ryan.
The lead didn’t last long. Brighton played a short corner and Leandro Trossard fired a low ball into the box, which bobbled around. It bounced off Rob Holding, who couldn’t clear it, and Dunk just about managed to poke it in.
Eddie Nketiah came on for the final 13 minutes but it was Aubameyang who went closest to the winner with a swerving effort that nearly caught out Ryan.
A late triple substitution saw Tierney, Reiss Nelson and Joe Willock introduced but it was Maupay who had the final impact with a calm finish in the final seconds.