Manchester United's Champions League hopes suffered a setback as Brighton scored a dramatic 99th-minute penalty winner to keep their own European ambitions on track.
Alexis Mac Allister converted deep into injury time to settle a Premier League thriller at Amex Stadium.
United - in the fourth and final Champions League qualification spot - stay four points ahead of fifth-placed Liverpool, but now have only one game in hand.
And Brighton deserved this victory for a relentless approach that finally gave them the breakthrough seconds from the end after Luke Shaw handled, the penalty awarded after a video assistant referee review.
Brighton, in sixth, are now only four points adrift of Liverpool, who have played two games more, and their celebrations at the final whistle contrasted sharply with the desolation of their defeat by United on penalties in the FA Cup semi-final at Wembley less than two weeks ago.
Both sides had chances in a superb game, with Antony shooting wide for United in the opening seconds while both Marcus Rashford and Antony Martial forced Brighton keeper Jason Steele into important saves.
Brighton, fresh from the 6-0 thrashing of Wolves, played with the verve that has become their trademark. Kaoru Mitoma shot straight at David de Gea when clean through, and missed another good chance after the break, while Danny Welbeck was also off target with another presentable opening.
The Seagulls turned up the pressure in stoppage time, De Gea saving superbly from Mac Allister and Pervis Etupinan before they finally made the breakthrough.
Superb Brighton aim for Europe
Brighton's thrilling attacking style under manager Roberto de Zerbi has got their fans dreaming of European football next season and the scenes of celebration at the final whistle reflected the belief and optimism surging through the club.
On this evidence, if Brighton do reach Europe, they will be a wonderfully entertaining addition.
They showed they have recovered from losing to United at Wembley by refusing to take a backward step, hunting down their tiring opponents in the game's later stages.
Brighton play with a pace and intensity that is pure theatre at times and in the end it just proved too much for United, although this game was settled on the fine margin of that late penalty.
Argentina World Cup winner Mac Allister ignored the noise and pressure to beat De Gea convincingly and send the Brighton fans wild. How they enjoyed De Zerbi leading his triumphant players on a lap of honour at the final whistle.
For all Brighton's excellence, they still relied heavily on goalkeeper Steele, who was outstanding when called upon.
Brighton have struggling Everton at home next and in this mood and form, will feel capable of beating anyone.
Man Utd away day blues continue
Manchester United are still in a good position to qualify for the Champions League by finishing in the top four but there is no doubt an indifferent away record is putting them under pressure.
This defeat was their seventh in 17 games, winning seven and drawing three, and for all they made Steele work, the traffic flowed nearly all one way towards their goal as the clock ticked down.
Shaw looked a very guilty man as Brighton manager De Zerbi and his entire backroom staff demanded a penalty in the closing seconds, the inevitable confirmed after VAR invited referee Andre Marriner to visit the screen.
United had their chances but found Steele in top form, their problems coming as they appeared to run out of steam in the second half.
Martial missed one of those opportunities but otherwise gave a lifeless, lacklustre performance and it was no surprise when he was eventually substituted.
United now travel to West Ham United on Sunday and will know their margin for error has been reduced by this defeat.
Player of the match
GilmourBilly Gilmour
Brighton & Hove Albion
Squad number27Player nameGilmour
Squad number10Player nameMac Allister
Squad number23Player nameSteele
Squad number5Player nameDunk
Squad number25Player nameCaicedo
Squad number30Player nameEstupiñán
Squad number7Player nameMarch
Squad number20Player nameEnciso
Squad number4Player nameWebster
Squad number22Player nameMitoma
Squad number40Player nameBuonanotte
Squad number21Player nameUndav
Squad number18Player nameWelbeck
Squad number6Player nameColwill
Line-ups
Brighton
Formation 4-2-3-1
23Steele
25Caicedo4Webster5Dunk30Estupiñán
27Gilmour10Mac Allister
40Buonanotte20Enciso22Mitoma
18Welbeck
23Steele25CaicedoBooked at 87mins4WebsterBooked at 52mins5DunkBooked at 70mins30Estupiñán27GilmourSubstituted forColwillat 75'minutesBooked at 77mins10Mac Allister40BuonanotteSubstituted forMarchat 63'minutes20Enciso22Mitoma18WelbeckSubstituted forUndavat 75'minutesSubstitutes
1Sánchez6Colwill7March21Undav26Ayari29van Hecke42Offiah49Moran51PeupionMan Utd
Formation 4-3-3
1de Gea
29Wan-Bissaka2Lindelöf23Shaw20Dalot
17Fred18Casemiro8Bruno Fernandes
21Antony9Martial10Rashford
1de Gea29Wan-Bissaka2Lindelöf23ShawBooked at 90mins20DalotBooked at 59mins17FredSubstituted forSabitzerat 76'minutes18CasemiroBooked at 44mins8Bruno Fernandes21AntonyBooked at 70minsSubstituted forSanchoat 76'minutes9MartialSubstituted forWeghorstat 84'minutes10RashfordSubstitutes
5Maguire12Malacia14Eriksen15Sabitzer25Sancho27Weghorst28Pellistri31Butland33WilliamsReferee:Andre MarrinerAttendance:31,577Live Text
Match ends, Brighton and Hove Albion 1, Manchester United 0.
Second Half ends, Brighton and Hove Albion 1, Manchester United 0.
Goal! Brighton and Hove Albion 1, Manchester United 0. Alexis Mac Allister (Brighton and Hove Albion) converts the penalty with a right footed shot to the top left corner.
VAR Decision: Penalty Brighton and Hove Albion.
Penalty conceded by Luke Shaw (Manchester United) with a hand ball in the penalty area.
Corner, Brighton and Hove Albion. Conceded by Wout Weghorst.
Luke Shaw (Manchester United) is shown the yellow card for a bad foul.
Julio Enciso (Brighton and Hove Albion) wins a free kick in the attacking half.
Foul by Luke Shaw (Manchester United).
Attempt saved. Pervis Estupiñán (Brighton and Hove Albion) left footed shot from outside the box is saved in the bottom left corner. Assisted by Solly March.
Attempt missed. Adam Webster (Brighton and Hove Albion) header from the centre of the box is just a bit too high. Assisted by Solly March with a cross following a corner.
Corner, Brighton and Hove Albion. Conceded by David de Gea.
Attempt saved. Alexis Mac Allister (Brighton and Hove Albion) left footed shot from outside the box is saved in the centre of the goal. Assisted by Solly March.
Foul by Julio Enciso (Brighton and Hove Albion).
Luke Shaw (Manchester United) wins a free kick in the defensive half.
Lewis Dunk (Brighton and Hove Albion) wins a free kick in the defensive half.
Foul by Wout Weghorst (Manchester United).
Moisés Caicedo (Brighton and Hove Albion) is shown the yellow card for a bad foul.
Attempt missed. Solly March (Brighton and Hove Albion) left footed shot from outside the box is close, but misses to the right. Assisted by Levi Colwill following a fast break.
Attempt blocked. Marcel Sabitzer (Manchester United) right footed shot from outside the box is blocked. Assisted by Casemiro.
Our coverage of your Premier League club is bigger and better than ever before - here's everything you need to know to make sure you never miss a moment