Pierre-Emerick Aubameyang scored his second goal in an Arsenal shirt but that was scant consolation on a miserable day on the south coast.
We started brightly enough but the gloom descended when Shane Duffy beat Petr Cech to a deep corner to send a header back across goal, and Lewis Dunk poked in the loose ball.
It went from bad to worse when Laurent Koscielny gave possession away with a wayward pass out of defence. Brighton worked the ball to Pascal Gross and he crossed for Glenn Murray to bury a downward header.
Cech kept the score down with a vital stop from Anthony Knockaert but Brighton looked dangerous every time they came forward and were dominant in the air.