Ashley Barnes' goal in first-half stoppage time had given the league champions the lead, putting Southampton's automatic promotion challenge in jeopardy.
However, a late surge stunned Brighton and gave Nigel Atkin's team a vital three points. David Connolly tied the score before Jose Fonte grabbed the winner in injury time.
Southampton may look back on this result in the summer and pinpoint it as the turning point that could see the Saints playing in The Championship next season.
There was little time for the Saints to relax as they faced Hartlepool on Easter Monday. However, a professional performance from Southampton claimed another three points with a 2-0 win.
A penalty from Ricky Lambert and a late strike from Fonte saw off Hartlepool, but the Saints remain level on points with Huddersfield in the League One table.
Huddersfield, who haven't lost since Southampton thrashed them in December picked up six points of their own over the long weekend. Both teams are on 83 points, but Saints have a better goal difference and a game in hand.
Southampton's extra game takes place next Monday against strugglers, Plymouth Argyle. A win for the Pilgrims could take them out of the danger zone, so expect Plymouth to take the game seriously.
Both Huddersfield and Southampton must play Brentford before the end of the season, but the big game this Saturday sees Huddersfield host Brighton. If Huddersfield fail to win and Saints can take another six points this weekend, they could go into the final weekend knowing promotion is secured.
There is a lot of football left to play, and Saints need to be at their best to secure a place in The Championship. More importantly, Southampton need points no matter how they come.
