Glenn Murray's crisp finish sparked a 19-minute goal blitz as Brighton stunned a defensively shoddy Manchester United 3-2 at the Amex Stadium.
Target-man Murray swept home his first goal of the season to toast his 200th Brighton appearance, before Shane Duffy and Pascal Gross left United reeling.
Romelu Lukaku found the net amid Brighton's first-half raids, but when Eric Bailly upended Gross to gift the hosts a penalty the Red Devils were effectively beaten by half-time.
Captain Paul Pogba struck a stoppage-time penalty for United but Brighton had done enough to earn a deserved victory.
United boss Jose Mourinho will doubtless be furious with his side's early defensive disarray - then later their inability to launch a fightback.
A surly Mourinho spent the build-up to this clash choosing to criticise rivals Manchester City's fly-on-the-wall documentary.
United's boss hit out at City, claiming "you can't buy class" in reaction to Amazon's behind-the-scenes account of last season at the Etihad Stadium.
At Brighton on Sunday however, the commodity United most needed to fork out for was imagination.
When Brighton's fans chorused, "Your city is blue" at the death, United and Mourinho's humiliation was complete.
This was just the 10th time in Premier League history that United leaked three first-half goals, and the first time since their 3-0 defeat at Arsenal in October 2015.
Brighton toppled United 1-0 on the south coast in May en route to a 15th-place finish in last term's Premier League. Just three months down the line, the Seagulls were at it again, with Gross - who hit the winner against United last term - instrumental again.
Lukaku's snatched, wayward shot was all United had to show for a wretched opening half-hour, where Mourinho's men crumbled under the weight of consistent home pressure.
Murray conjured a tidy finish to put the hosts ahead, the 34-year-old flicking home with one touch to send the home fans into raptures.
And just two minutes later the Seagulls supporters were on their feet again as Duffy slotted in after United failed to clear their lines.
Lukaku made no mistake with his second chance however, heading home from close range.
But just when United expected to force their way back into the contest, more poor defending cost them again.
Bailly upended Gross from behind to concede a spot-kick that the Austrian then converted.
Brighton took a healthy 3-1 lead into the break and Mourinho used the interval to hook Juan Mata and Andreas Pereira.
England men Marcus Rashford and Jesse Lingard joined the fray with United desperate for an injection of pace and incision.
Mourinho then threw on Marouane Fellaini in place of Anthony Martial on the hour, but still United continued in listless fashion.
Pogba converted his second penalty in as many games after Duffy upended Fellaini, but it was too little, too late for United.
Mourinho could inspire no comeback from his faltering charges, and the ex-Chelsea boss will now demand an instant reaction when United host Tottenham on Monday, August 27.
- Since taking charge of Manchester United at the start of the 2016/17 season, the only opponent Jose Mourinho has lost against more often than Brighton (2) is his former club Chelsea (4).
- Brighton have lost just one of their last eight home games in the Premier League (W5 D2 L1), with the two most recent both being wins against Manchester United (1-0 in 2017/18 and 3-1 in 2018/19).
- Manchester United lost their first away game of a Premier League campaign for the first time since 2012/13, when they lost 0-1 against Everton at Goodison Park.
- 23 of Glenn Murray’s 24 goals in the Premier League have been scored from inside the box, including 100% of those scored for Brighton in the competition (13/13).
- Pascal Groß has scored eight goals in the Premier League, with all eight coming at home. Only two players have netted more goals exclusively at home in the competition’s history – Georginio Wijnaldum (18) and Clive Wilson (9).
- In competitive games since the start of the 2018 World Cup, David de Gea has conceded 10 goals from his last 14 shots on target faced for Manchester United and Spain combined.
- Romelu Lukaku has now scored against 18 of the current 20 Premier League teams, with Cardiff and Wolves the only teams he is yet to find the net against.
- Paul Pogba ended on the losing side in a Premier League game he scored in for the very first time – prior to today, Manchester United had won each of the previous 11 games he had netted in, which was a joint-record in the competition (with Ryan Babel for Liverpool – 11 games).
- This was only the tenth time in Premier League history that Manchester United conceded three first half goals in a game, and the first time since October 2015 v Arsenal.