He scored a golden goal and ended Birmingham's barren Easter spell when they were defeated by both Burnley and Barnsley.
The success was sealed in the 78th minute when veteran Andy Cole cleverly set up Nicklas Bendtner to calmly score the second goal.
Birmingham boss, Steve Bruce, was relieved the Blues had been able to keep up the promotion challenge in their last but one home game of the season.
Bruce celebrated his 250th game in charge at Birmingham with a crucial win which keeps the promotion race at boiling point.
Southampton, however, suffered their second successive defeat which leaves them just outside the play-off zone.
The game was far from a classic and Jaidi's 33rd minute strike was the first time either side had managed to get an effort on target.
Only minutes earlier Birmingham had suffered a set-back when they lost their skipper, Damien Johnson, with a hamstring injury.
They took time to re-organise and seconds before Jaidi's vital goal Danny Guthrie produced a powerful low drive which rattled against the base of the post.
Birmingham went away and in their next attack won a corner on the left. Gary McSheffrey's high ball into the middle was met majestically by Jaidi to power home his header into the roof of the net.
Southampton, however, were sadly out of luck at a vital stage of the season.
They lived dangerously in the second half and were indebted to a fine save by Bartosz Bialkowski to prevent Bendtner increasing Birmingham's lead.
From the corner the on-loan striker from Arsenal rammed a header against the bar Birmingham, but he made no mistake in the 78th minute when he jinked his way around Bialkowski to notch his tenth goal of the season.
Marek Saganowski, who was, effectively shunted out the game by Martin Taylor and Jaidi, raised Southampton's hopes with a 84th minute goal to take his tally to ten in ten games, but it was no avail as the Saints were foiled in their bid to complete the double.