The Blues had twice fallen behind to Southampton, who have already guaranteed themselves a place in Europe by reaching the FA Cup final, but emerged victorious.
Not for the first time it was mercurial Frenchman Christophe Dugarry who was the hero for the home side, claiming a brace that included the all-important winner.
Birmingham had already had a penalty claim dismissed after Michael Svensson had felled Robbie Savage in the box when, against the run of play, Anders Svensson had curled home an opener for Southampton.
Dugarry levelled matters with a brilliant free-kick, although only on his second attempt, with James Beattie ajudged to have encroached in blocking his first effort.
Brett Ormerod put Saints back ahead when he smashed home a Svensson flick-on, to keep up his recent good scoring run.
However, Bryan Hughes levelled once more for The Blues when he volleyed home after good work from Geoff Horsfield.
It was Dugarry that wrapped up victory when he headed home with aplomb, to the delight of the vociferous home crowd.
