Arsenal's challenge to return to the Champions League next season faltered once more as Neal Maupay's 95th minute winner handed Brighton a vital 2-1 win in their battle to beat the drop.
Nicolas Pepe's stunning strike gave the visitors a deserved lead at the Amex, but the Gunners defensive problems reared their head again as Lewis Dunk bundled home from a short corner.
Maupay had angered the Arsenal players for his part in a serious injury suffered by goalkeeper Bernd Leno in the first half.
And the Frenchman rubbed salt into those wounds with a fine finish deep into stoppage time to snatch all three points.
A second defeat in four days leaves Arsenal still eight points outside the top four in ninth, having played a game more than fourth-placed Chelsea.
A first league win of 2020 takes Brighton five points clear of the drop zone in 15th.