Birmingham virtually guaranteed Premiership football for a second season as two-goal hero Christophe Dugarry inspired them a thrilling victory over Gordon Strachan's side.
Blues' sixth win in eight games took them up to the 44 point mark and almost out of sight of the relegation zone.
Steve Bruce's side produced some of their most fluent football of the season in the opening 45 minutes and yet found themselves in arrears.
Dugarry was at his imperial best and tormented the Saints with his skill and trickery and it is no wonder that Bruce wants to make permanent his move from Bordeaux.
But Southampton, with the aid of some fine saves from Paul Jones, hung in there before taking the lead against the run of play.
In the second half Gordon Strachan's side started to play with more purpose but Bruce's side now showed the fighting qualities and twice came from behind to draw level - before fittingly Dugarry headed the winner seven minutes from time.
Blues defender Kenny Cunningham got in an important early block on a shot from Saints striker James Beattie and, when the ball looped up to David Prutton, he sent his header across the face of Ian Bennett's goal.
But the home side were soon into their stride and Jones produced a fine save to turn a curling effort from Bryan Hughes around the post after he had cut in from the right flank.
Then Dugarry failed by inches to get on the end of an inswinging free-kick from Stephen Clemence after Geoff Horsfield had been brought down by Michael Svensson.
Robbie Savage appealed in vain for a penalty to referee Steve Bennett after a challenge on him by Michael Svensson after he had been put clear by Matthew Upson.
The on-loan Dugarry was looking dangerous and Claus Lundekvam knew nothing about a powerful volley which struck him on the back and flew out of play for a corner.
Dugarry was a problem for the Saints every time he ventured forward and Jones again was extended as he beat out a stinging drive from the French World Cup winner.
But against the run of play Southampton went ahead after 25 minutes through a goal from Anders Svensson - his second in 48 hours.
Beattie flicked the ball through to Svensson, on target in Saturday's win over Leeds, who then turned it past Bennett from a narrow angle on the left side of the penalty area.
Saints now started to show their quality and Beattie wasted a golden chance to score his 24th goal of the season when he headed wide when unmarked from six yards out from a Fabrice Fernandes free-kick.
The England striker was then denied by a fine block from Bennett after Anders Svensson had played the ball into his path following a mistake by Dugarry after he had been over-elaborate.
Wayne Bridge became the first player to be yellow-carded after 43 minutes for dissent after kicking the ball away.
A testament to the way they played was that Blues received a standing ovation when they went off at half-time despite the scoreline.
Strachan made a half-time substitution, bringing on Brett Ormerod for Rory Delap to partner Beattie with Anders Svensson moving back into midfield.
Prutton made a good break forward to get onto the end of a low cross from Paul Telfer but his shot was a weak effort which was comfortably dealt with by Bennett.
Bruce opted for a change after 54 minutes, bringing on Stan Lazaridis for Clemence.
But it was Saints who came close to doubling their lead when Bennett finger-tipped away a dipping effort from Anders Svensson.
Dugarry did have the ball in the net after 56 minutes but play had already been halted for a foul on Jeff Kenna.
Bennett did well to parry away a dipping long range effort from Anders Svensson who had plenty of time to line up his shot.
Bruce made a second change after 65 minutes with Paul Devlin replacing Kenna but it needed Bennett to be at his best to deny Beattie at close range.
Blues finally equalised after 74 minutes with Dugarry's 25 yard free-kick containing too much power for Jones even though he got a hand to the ball.
Southampton's reply was instant and Ormerod thundered a shot past Bennett into the roof of the net after a flick on by Michael Svensson.
But Blues stormed back and Hughes made it 2-2 with a volley from a cross by the tireless Horsfield.
Then in the 83rd minute St Andrews erupted when Dugarry climbed at the far post to head in a centre from substitute Stern John.
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