Brighton took the lead at the Emirates Stadium when Leandro Trossard turned home a low cross from Enock Mwepu to finish a good move from the visitors.
Arsenal had struggled to create chances but thought they had levelled through a close-range effort from Gabriel Martinelli, only for a lengthy VAR check to cancel out his header.
Brighton doubled their lead through Mwepu’s low drive from the edge of the box following a neat interchange between Trossard and Moises Caicedo.
Arsenal gave themselves hope of salvaging a result as Martin Odegaard’s speculative effort looped up off Danny Welbeck to half the arrears.
The result is a huge blow for Arsenal's Champions League hopes an they have now lost two on the bounce following their 3-0 loss at Crystal Palace last Monday.
For Brighton it was their first win since their 2-0 triumph at Watford way back in February. The win lifts them to 11th in the league standings and in with a chance of achieving their aim of a top 10 finish.
Sonny Turner was at the Emirates Stadium for Sussex World and here's his player ratings
9. Alexis Mac Allister- 7
Moved the ball nicely and was a positive force in transitions for the visitors. Didn’t see a lot of the ball, but showed good feet, and some nice flicks to move Brighton into dangerous areas.
10. Leandro Trossard- 8
Opened the scoring with a fantastic finish, arriving just inside the area to fire the ball into the top corner. Showed threat all game, but crucially for the Seagulls showed a willingness to help out defensively, a lovely piece of skill under pressure, backheeling the ball through Odegards legs to relieve the Arsenal pressure was neat defensive work.
11. Danny Welbeck- 5
A lack of service for the Brighton striker hindered his performance. While he typically worked hard up front for the team, he didn’t manage to get himself into a goalscoring opportunity, and lacked threat.
12. Adam Lallana- 6
Helped in midfield in a frantic finish to the game, showing good energy in closing down Arsenal and making it as difficult as possible to create an equaliser.