Pascal Gross scored the goal which sank Manchester United 1-0 to guarantee Premier League football for Brighton next season.
The German midfielder, Albion's undisputed player of the year, fittingly hit his seventh goal of the season to take the Seagulls to the magic 40-point mark.
United arrived on the south coast knowing a win would all-but seal second place for them, but a desperate disappointing display means they will have to wait to secure the runners-up spot.
But Chris Hughton's side can now look forward to another season in the top flight having assured they are mathematically safe ahead of trips to Manchester City and Liverpool.
It is some achievement for Hughton, who guided Brighton into the Premier League for the first time last season, and for a club who 21 years ago this week needed a late equaliser at Hereford to save themselves from dropping out of the Football League altogether.
Jose Mourinho did make six changes to a United side which was missing Romelu Lukaku, as expected, but the absence of Alexis Sanchez, also through injury, set alarm bells ringing 15 days before they play Chelsea in the FA Cup final.
It meant rare chances in attack for Marcus Rashford and Anthony Martial in a lineup which also included Sunday's match-winner against Arsenal, Marouane Fellaini.
The Belgian, whose future remains uncertain with his contract almost up, had the ball in the net after only three minutes but he was offside when he turned home Rashford's free kick.
That was about as good as it got in a horribly disjointed display from United.
Paul Pogba was back to his laid-back worst and Juan Mata's passing was horribly out of kilter.
Brighton began to sense it might be the night to get the win they needed and David De Gea had to be alert to keep out a low Gross shot.
De Gea was then almost caught out by a dipping 25-yard effort from Glenn Murray which he pushed away at full stretch, before the Spaniard tipped Jose Izquierdo's fierce drive over and denied Gross again at his near post.
The hosts, aside from one Lewis Dunk slip which Rashford was unable to take advantage of, were well on top but could not find the breakthrough before half-time.
But Brighton got the goal they deserved in the 56th minute when Izqueirdo skipped past Matteo Darmian and stood a cross up towards the far post.
De Gea pushed the ball onto the head of Gross and although Marcos Rojo hooked it clear from underneath the crossbar goalline technology confirmed it had crossed the line.
United belatedly rallied but Maty Ryan saved from Rashford and Jesse Lingard fired wide as Brighton held out for a memorable and thoroughly merited win.