Southampton have finished as runners-up in Premier League 2 Division 2 after their 5-1 home win against Sunderland was not enough to stop Wolverhampton Wanderers being confirmed as champions on goal difference.
Requiring a 12-goal win to replace Wolves in the only automatic promotion spot, Saints fell behind in the fourth minute when Lee Connelly scored for Sunderland.
But Christoph Klarer headed in Saints' leveller and Tyreke Johnson gave the hosts a 2-1 half-time lead at St Mary's Stadium.
William Smallbone converted a 54th-minute penalty to make it 3-1, substitute Marcus Barnes netted a fourth and Alfie Jones completed the scoring on 70 minutes.
Saints' victory moved them up to 43 points with a goal difference seven inferior to Wolves, whose 3-2 comeback win at Manchester United on Friday proved decisive in the title race.
Saints will host fifth-placed Aston Villa in their playoff semi-final, a one-off match at St Mary's on Friday 3 May at 19:00 BST. The winners will face Reading or Newcastle United, who also meet at 19:00 BST on Friday 3 May at the Madejski Stadium, for promotion in the playoff final.
Sunderland end the season at the bottom of the table with 12 points.