What you need to know:
The 1-0 win completes a league double for the Seagulls against United, who beat Erik ten Hag's men in their first match of the season but suffered a painful defeat on penalties in last month's FA Cup semi-final at Wembley
Brighton gained sweet revenge on Thursday for their defeat by Manchester United in the FA Cup semi-finals, with Alexis Mac Allister converting a 99th-minute penalty to dent the visitors' Champions League hopes.
Classy Brighton dominated possession but lacked a cutting edge until Luke Shaw's handball deep into stoppage time presented them with an opportunity to take all three points.
The 1-0 win completes a league double for the Seagulls against United, who beat Erik ten Hag's men in their first match of the season but suffered a painful defeat on penalties in last month's FA Cup semi-final at Wembley.
United remain in the fourth and final Champions League qualification spot in the Premier League, four points ahead of fifth-placed Liverpool with a game in hand, while Brighton are up to sixth.
"There is a God of football," Brighton boss Roberto De Zerbi told Sky Sports.
"We deserved to win the semi-final but lost on penalties but today we won with a penalty. In both games I think we played better than United."
Ten Hag bemoaned his team's lack of finishing, but said United's destiny was still in their own hands.
"We have to finish the chances, be more clinical and don't make the mistakes we did like in the last seconds," said the disappointed Dutchman.
"We have everything in our hands and we have to bounce back. We take the lessons from tonight and we have to move on."
Both teams squandered clear-cut chances in the opening minutes of an entertaining encounter at the Amex Stadium.
United's Brazilian winger Antony was put through by Bruno Fernandes in just the second minute but fired wastefully wide.
Two minutes later Japan's Kaoru Mitoma intercepted a careless ball across the defence at the other end and broke into the box before hitting a shot that smashed goalkeeper David de Gea in the face.
The lively Mitoma, putting United right-back Aaron Wan-Bissaka under huge pressure, danced into the area again in the 13th minute but was unable to finish.
The home side, bursting with confidence after a 6-0 win against Wolves, were enjoying the bulk of possession but United looked dangerous on the break.
The impressive Marcus Rashford hit a shot from a tight angle that Brighton goalkeeper Jason Steele did well to keep out at his near post.
Argentine World Cup winner Mac Allister shot wide of De Gea's goal while Anthony Martial's effort was pushed away by Steele at the other end.
De Zerbi's team were again on top in the early stages of the second half but Mitoma and Danny Welbeck were guilty of wasting openings.
As the half wore on the game became more scrappy, suiting United, who were struggling to find fluency, but Brighton finished the stronger of the two teams.
In the 90th minute De Gea produced a smart save to keep out a Mac Allister strike, puffing out his cheeks in relief.
But the home side had one final chance after Shaw was penalised for handball following VAR intervention and Mac Allister made no mistake, lashing the ball high into the net to spark wild celebrations on the south coast.