Chris Hughton's side beat Wigan on Tuesday (AEST) to stay top of the Championship and put a 15-point gap between themselves and fourth-placed Huddersfield Town.
Huddersfield then failed to defeat Derby County in the later match – Jacob Butterfield netting a late leveller for the Rams – meaning Brighton's top-two finish is mathematically assured with three games left to play.
It means the Seagulls will be playing top-flight football next season for the first time in 34 years.
The south-coast side are closing in on the Championship title, too, with Newcastle United's 2-1 defeat to Ipswich Town leaving Rafa Benitez's side seven points behind in second.
Three points from their remaining matches against Norwich City, Bristol City and Aston Villa will see Brighton crowned champions.
It marks an incredible turnaround for a club who were almost relegated from the Football League in 1997.