Brighton piled the misery on Arsenal with a hard-fought and deserved 2-1 win at the Emirates Stadium, where boos rung out at full-time in interim boss Freddie Ljungberg's first home game in charge.
The visitors had never previously won away at Arsenal but they dominated the first half and withstood an Arsenal assault after the break before Neal Maupay headed home the decisive goal on 80 minutes.
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Alexandre Lacazette (50) had earlier cancelled out Adam Webster's (36) opener with a bizarre flicked header from a corner but Arsenal - who had a David Luiz strike rightly ruled out by VAR for offside at 1-1 - couldn't find another way through, with Mat Ryan making a fine stop late on to deny substitute Gabriel Martinelli.
Arsenal, who have now gone nine games without a win in all competitions, remain down in 10th place in the Premier League and trail fourth-placed Chelsea by 10 points. Brighton, meanwhile, have pulled four points clear of the relegation zone and are up to 13th
How Brighton pulled off historic win
Aaron Connolly almost capitalised on a mix up between David Luiz and Bernd Leno inside 60 seconds and that set the tone for the first 45 minutes.
While Arsenal occasionally threatened through Pierre-Emerick Aubameyang down their right flank, the majority of the first half was played out in the hosts' half, with Maupay fizzing a dangerous cross into the six-yard box which Connolly just failed to reach before the Frenchman forced Leno into a smart save.
Brighton - a real threat from set-pieces - won five corners in the first half and made their dominance count on 36 minutes when Webster reacted quickest to the loose ball in the box and slammed home.
Arsenal responded with Joe Willock testing Ryan with a good header before Lacazette rightly had his penalty appeals turned away when he went down under a challenge from Webster.
Arsenal brought record signing Nicolas Pepe on at half-time in place of Willock and their intensity was transformed after the re-start. Within five minutes they were level.
Pepe probed down the right and won a corner and Lacazette's flick-on from Mesut Ozil's set-piece crept in at the back post while Sead Kolasinac battled to turn it home. There was a VAR check on Kolasinac's challenge on Davy Propper but after a nervy wait Arsenal's equaliser was confirmed.
Arsenal: Leno (6), David Luiz (5), Kolasinac (6), Bellerin (6), Sokratis (6), Xhaka (6), Willock (5), Ozil (6), Aubameyang (7), Torreira (6), Lacazette (7).
Subs: Tierney (6), Pepe (7), Martinelli (N/A).
Brighton: Ryan (8), Webster (8), Stephens (7), Dunk (7), Propper (7), Burn (7), Mooy (8), Alzate (7), Maupay (9), Gross (7), Connolly (8).
Subs: Duffy (N/A), Montoya (6), Trossard (n/a).
Man of the match: Neal Maupay (Brighton)
Brighton still had their chances, with Maupay just failing to connect from close range on 61 minutes but moments later David Luiz had a goal ruled out for offside by VAR, when he was judged to have gone too soon before sweeping home from a free-kick.
Arsenal were on top and seemed set to go on to win but Brighton impressively rode the storm and continued to cause their hosts problems.
Maupay - a thorn in Arsenal's side all night - had a shot blocked by Sokratis on 79 minutes but then just moments later had the ball in the back of the net, pulling away from David Luiz to head home Aaron Mooy's centre.
As the Arsenal home fans began to vent their anger, teenager Martinelli tested Ryan with a header and Pepe had a shot deflect wide but Brighton saw out the match to claim a deserved, valuable and historic three points.
Arsenal interim manager Freddie Ljungberg made three changes to the team which drew 2-2 at Norwich, with Shkodran Mustafi coming out of the squad completely, while Calum Chambers and Matteo Guendouzi dropped to the bench. In came Hector Bellerin, Sokratis and Lucas Torreira.
Brighton made two changes to switch to a back three, with Steven Alzate and Neal Maupay coming in for Martin Montoya and Yves Bissouma.
What the managers said...
Arsenal interim head coach Freddie Ljungberg: "We have to keep going. The players are low on confidence, I think you can see that. We had a bit of a reaction at the second half but we have to start the game like that, not at half-time. That's the biggest disappointment.
"We are in a difficult situation. We have lost a lot of games, the confidence is down. That's the main thing. If you looked at how we started the game - no energy, we were very flat.
"It's my job to get them more aggressive or have more energy but I think it's the confidence. A lot of things to think about."
Brighton boss Graham Potter: "We showed great quality and great courage, so overall I'm delighted with the players. It's fantastic to get three points and play like we did.
"Given the quality they have up front everything can change, but credit to the players, they managed the situation well, and there was a bit of quality from us as well.
"We've had a tough batch of games so it's nice to come away from home, win, and play well. It gives us a bit more confidence on the path to going somewhere."
Analysis: Arsenal's pain continues
Sky Sports' Oliver Yew at the Emirates Stadium…
For whoever comes in at Arsenal to replace Unai Emery on a full-time basis, the job at the Emirates Stadium is getting even bigger - and doesn't Ljungberg, who is currently in interim charge, know it!
As early as the first minute it was plain to see that the confidence in this Arsenal team was on the floor as a mix-up between Leno and David Luiz almost gifted Brighton an early lead. Ljungberg was up and down like a yo-yo as he looked to cajole his side into life, but he could only watch on as his team gave the ball away time after time after time. The Gunners were woeful, and they got what they deserved when Webster gave the visitors the lead. In truth, Arsenal were lucky not to be further behind at the break.
Attention quickly turned to Ljungberg's half-time team talk and whatever he said to his troops at the break did the trick - for a while. He also threw on Pepe at the break and the Ivory Coast international immediately injected some much-needed urgency into Arsenal's attacking play, but they soon slipped back into old habits.
Brighton continued to cause problems from set-pieces and on the break as Arsenal continued to stand off their opponents, and they eventually exposed Arsenal's defensive frailties to get their just rewards through Maupay's late winner.
It's another huge blow for Arsenal. The result is also a massive dent to Ljungberg's aspirations of getting the job on a full-time basis. Arsenal's search for a new head coach has taken an urgent turn.
- Arsenal are now winless in nine games in all competitions, their worst run since going 10 matches without victory between February and March of 1977 under Terry Neill.
- Brighton have beaten 'big six' opposition away from home in the Premier League for the very first time at the 17th attempt; they had lost 15 of the previous 16 such games.
- Arsenal have faced 52 shots on target in Premier League home games this season - in the entire Invincibles season in 2003-04, they allowed just 48 opposition shots on target at home.
- Only Norwich (10) have conceded the first goal in more Premier League games than Arsenal this season (9).
Arsenal travel to West Ham on Monday Night Football, which you can watch live on Sky Sports Premier League from 7pm; kick-off is at 8pm.
Meanwhile, Brighton host Wolves on Super Sunday, which you can watch live on Sky Sports Premier League from 4pm; kick-off is 4.30pm.
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