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
5. Moises Caicedo- 7
Did extremely well to spot Mwepu and dig out a pull back as he was falling over to set up Brighton’s second. Had an assured game, and like his fellow midfielders showed good fight and energy against a lot of pressure.
6. Enock Mwepu- 10
Brighton’s best player. Showed great vision to find Trossard on the edge of the penalty area who fired home Brighton’s opener. Then added to the goal tally with a fantastic volleyed finish, controlling the ball with his side-foot to guide into the bottom corner. Busy in midfield, and was the energy Brighton have been missing in recent weeks.
7. Pascal Gross- 6
Not his best game, but played his part in helping Brighton to survive the immense pressure put on them. Showed an attacking intent when on the ball.
8. Yves Bissouma- 7
Was good cover in front of the defence, crucially dispossessing Lacazette in the first half who had turned Dunk, with the scores still level. Partnered Mwepu well and was combative in midfield for the Seagulls.