Julio Enciso's 51st-minute header broke the deadlock before former Brighton striker Leandro Trossard gifted Deniz Undav the chance to double their lead on 86 minutes. As the game went into stoppage time, Pervis Estupinan added a third as he turned home the rebound after Gunners goalkeeper Aaron Ramsdale saved Undav's long-range effort.
Roberto De Zerbi's side earned a precious win, which keeps their pursuit of European qualification on track while leaving the Gunners' chances of toppling Manchester City hanging by a thread.
A cagey first half gave way to a more open, free-flowing second period in which Brighton played the better football and Arsenal struggled to replicate the form which had launched an improbable title challenge. Estupinan's initial cross was intercepted by Kieran Tierney but the Ecuadorian defender scuffed the rebound back across goal where Enciso was unmarked to head home.
As they chased an equaliser, Arsenal attempted to play out from the back but Trossard's first-time forward pass was intercepted by Pascal Gross and the loose ball fell perfectly for substitute Undav, who lifted a clever finish over Ramsdale.
Arsenal remain four points behind City having played a game more, meaning City will be crowned champions if the Gunners lose at Nottingham Forest next Saturday.
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1. Arsenal's title hopes have surely run their course
That is surely that. Arsenal play again before City but the gap looks too large, the opportunity lost. Beating Newcastle United last weekend appeared to breathe fresh life into the title race, but Sunday was surely the decisive day as City eased to a 3-0 win at Everton before Arsenal were beaten by the better team and the same scoreline in North London.
Brighton still have to play City but they are already the kingmakers, dealing Arsenal's title hopes the final, savage blow. Mikel Arteta admitted before the game there would be no way of keeping the City result from his players and perhaps the emphatic scoreline at Goodison Park contributed to a flat display in which they failed to truly rouse themselves even after falling behind.
It will rankle with Arteta that they were unable to take City right to the final week of the season but two wins from their last seven games has proved enough of a wobble to allow Pep Guardiola's side to surge past them into a near unassailable lead. There were no boos at the final whistle. Thousands of fans had left in disappointment but those who stayed cheered their team. They know how much they have given. But they also now know it won't quite be enough to win their first Premier League title since 2004.
2. Arsenal showed some fight but not their football
The most impressive aspect of last weekend's win at Newcastle was the manner in which Arsenal stood up to the physicality and ferocity of their opponents, earning the right to play as their quality came to the fore. Sunday pitched two of the most aesthetically pleasing teams against each other yet the first half was an attritional affair, exemplified by Gabriel Martinelli's rough challenge on Kaoru Mitoma returned in kind by Moises Caicedo's tackle on the Brazilian a few minutes later.
Arsenal created dangerous moments as Leandro Trossard hit the crossbar, Gabriel Jesus was denied by a smart save and Bukayo Saka flashed a shot wide on the stroke of half-time. But Brighton had 62% possession -- with also a marginally higher xG of 0.50 to 0.45 despite having three shots to Arsenal's eight -- and stepped it up after the interval, playing some sumptuous football seemingly able to perform free of the pressure which perhaps inhibited the home side.
It is testament to Arsenal's quality this season that they have so rarely been outplayed. The Gunners have now lost five league games this season. Manchester City taught them a lesson in both of their wins, but Manchester United's 3-1 win in September was a little smash and grab, as was Everton's
3. Injuries highlight Arteta's need to strengthen side
Arsenal reacted smartly to acquire Jorginho and Leandro Trossard in January when missing out on their preferred targets (Moises Caicedo stayed at Brighton while Chelsea outbid the Gunners for Shakhtar Donetsk winger Mykhailo Mudryk). Those two signings helped them stay with Manchester City for longer -- even as recently as last weekend, Jorginho excelled as Arsenal beat Newcastle 2-0.
But the next step for Arteta is to add further quality to match City's consistency, especially with Champions League football returning to Emirates Stadium for the first time in six years. Gabriel Martinelli was forced off injured after 20 minutes, joining William Saliba and Oleksandr Zinchenko on the touchline - three key absentees who were all sorely missed.
Martinelli was seen wearing a protective boot at full-time, suggesting he is a major doubt for their final two matches. These are, of course, fine margins given Arsenal have exceeded expectations this season but if they are to match City across multiple competitions next term and sustain another title challenge, Arsenal need to add depth in several areas: centre-back, central midfield and left back most obviously.
As Rob Holding has struggled deputising for Saliba, eventually losing his place to Jakob Kiwior, Kieran Tierney faltered in Zinchenko's absence, his meek header enabling Brighton to score their opening goal. Eddie Nketiah and Reiss Nelson have played their part at times this season but they were ineffective off the bench here while forgotten man Emile Smith Rowe's future looks uncertain.
In the bigger picture, Arsenal are ahead of schedule in their squad building but it is nevertheless clear what their next step should be.
Best and worst performers
BEST: Kaoru Mitoma (Brighton).
Had the beating of Ben White all afternoon with some sublime wing play.
BEST: Pervis Estupinan (Brighton).
A goal, an assist and a clean sheet from the all-action Brighton defender.
BEST: Levi Colwill (Brighton).
Did a fine job stifling Martin Odegaard and ended with 108 touches, completing 86.3 per cent of 95 passes attempted.
WORST: Jorginho (Arsenal).
Unable to cope with Brighton's movement in midfield and substituted for Thomas Partey on the hour mark.
WORST: Ben White (Arsenal).
Looked lost in the air and all at sea on the ground as Brighton targeted his channel.
WORST: Kieran Tierney (Arsenal).
Unable to influence play like Zinchenko in possession and struggled to provide defensive solidity.
Highlights and notable moments
Already down one, a mix-up in the back put Arsenal down by two.
After the match: What the managers and players said
Arsenal's Martin Odegaard on the title chase, to Sky Sports: "Yeah it feels like that [whether the title is over]. It is going to be very difficult now we have to be honest. It is tough to take ... It is not a good feeling at the moment. The way we played, particularly in the second half, I don't know what happened to be honest. It feels like there is no hope now."
Brighton striker Deniz Undav on the win, to Sky Sports: "The manager showed us a Michael Jordan video, to show us how he motivated himself each game. He showed us after the defeat by Everton, and I think it was the right choice to show us the video; we showed today how mentally strong we are."
Key stats (provided by ESPN Stats & Information research)
- Brighton had never beaten Arsenal by more than one goal, and had never held Arsenal scoreless in a Premier League match
- With this loss, Arsenal has nine points from its last seven league matches, the club's worst span of seven matches all season.
Arsenal: Heading to Nottingham Forest on Saturday, where a Gunners loss would hand Manchester City the title. A win or draw would keep Arsenal's slim hopes alive but depends on City's result against Chelsea the next day.
Brighton: A trip to Newcastle United on Thursday, where the Seagulls will look to beat another Champions League-bound side while boosting their own hopes of European competition next season.