Pascal Gross' second-half header guaranteed Brighton's Premier League status following a 1-0 win over a flat Manchester United on Friday Night Football.
Gross broke the deadlock with a close-range header which fell 2.8 centimetres over the line - confirmed by the Goal Decision System - on 57 minutes, after David de Gea's superb save from Glenn Murray's dipping volley kept the scores level at the break.
Both sides had chances in the second half to score the second, but United were well below-par as they look to finish the Premier League season on a high ahead of the FA Cup final against Chelsea.
Brighton: Ryan (7), Bruno (8), Duffy (8), Dunk (8), Bong (7), Knockaert (9), Stephens (8), Propper (7), Izquierdo (8), Gross (8), Murray (7)
Subs: Kayal (NA), Ulloa (NA), March (NA)
Man Utd: De Gea (7), Darmian (6), Smalling (6), Rojo (5), Young (5), Fellaini (6), Matic (5), Pogba (4), Mata (5), Rashford (6), Martial (5)
Subs: Lingard (6), Shaw (5), McTominay (NA)
Man of the match: Anthony Knockaert
The result means 11th-place Brighton are safe following their first Premier League season, while Jose Mourinho's United still have work to do to guarantee second place.
After Marouane Fellaini had seen an early goal correctly ruled out for offside, Brighton's Glenn Murray forced a superb save out of De Gea with a fine half-volley from 25 yards.
Get MUTV half price
Man Utd fans, get closer to the club you love with half price MUTV. Just £3.50 a month for 12 months
De Gea, who was named Manchester United Player of the Season for a record fourth time this week, did well to tip over Jose Izquierdo's long-range effort, and then showed his worth yet again by blocking Pascal Gross from an angle at the near post after some sloppy play by Fellaini.
Brighton made two changes, with Murray and Propper in for Ulloa and Kayal. Man Utd made six changes from the side that beat Arsenal 2-1 last weekend, with Rashford, Mata, Martial, Fellaini, Darmian and Rojo coming in. Sanchez and Lukaku were not involved.
De Gea's opposite number Mathew Ryan did well to stop a low Paul Pogba effort after the break, but Brighton took the lead on 57 minutes thanks to the help of the Goal-line Decision System.
Izquierdo's cross down the wing was not dealt with by United, allowing Gross to pile in from six yards out and head inches over the line, despite Marcos Rojo's best efforts to keep the ball out.
Izquierdo nearly doubled Brighton's lead moments later, but fizzed an effort just wide at the far post with United on the ropes and both Pogba and Anthony Martial, in particular, struggling to influence the game.
Martial did strike just wide towards the end, while sub Jesse Lingard miscued from a clear opportunity inside the box after a well-worked one-two with Martial.
Chris Hughton: "There's a group of players with great belief and discipline. We learned after the first few games that the performances were good enough that we weren't always going to get beat, and that gave us confidence."
Jose Mourinho: "Maybe now you understand why some players play more than others, and you don't ask all the time why A, B and C don't play more. I thought for some of them the possibility to start the game, to give reason to the ones who all season have been critical towards my choices.
"Probably we are not as good individually as people think we are, and I keep saying, if you manage to finish second, it is great, more than great if you manage to finish second. But now we need four points and we must try to get them."
Man of the match - Anthony Knockaert
"Not just for lighting up the stadium for his dribbles, but he's got into his defensive positions so well," said Gary Neville. "Both sides of his game were superb."
Knockaert constantly wanted the ball down the right and caused Ashley Young all manner of issues, with a team-high 57 touches and 19 duels.
England World Cup watch
Ashley Young was left exposed as most of Brighton's attacks came down United's left, while Chris Smalling did his chances no harm but didn't overly impress.
Marcus Rashford struggled to make an influence in the game as Jose Mourinho gave him a rare chance up front, while Jesse Lingard came off the bench in the second half.
Glenn Murray, who has a outsider's chance of making the squad if there are injury issues for England up front, held the ball up well and put in an impressive performance with just a week-and-a-half until the squad is announced.
- Brighton registered only their second win in 19 matches against Man Utd in all competitions, with this their first since a 1-0 victory in the top-flight in November 1982.
- Manchester United have lost away from home against three newly promoted sides in a league season for the first time in their history, also losing at Huddersfield (1-2 in October) and Newcastle (0-1 in February).
- It was also the first time they'd lost three matches against newly promoted sides in a league season since 1989/90, when they lost to Man City, Crystal Palace and Chelsea.
- Jose Mourinho has lost six of his last 10 away Premier League matches in the month of May (W1 D3).
- Pascal Gross has been directly involved in 45 per cent of Brighton's 33 Premier League goals this season (15 - 7 goals, 8 assists).
Live Premier League
United now go to West Ham on Thursday evening, live on Sky Sports Premier League, while Brighton visit champions Manchester City on Wednesday night.