The 34-year-old Tunisian international defender, who was released by Blues at the end of last season, has signed a two-year contract at St Mary's, subject to international clearance.
Jaidi made 93 appearances for Blues during his three-season spell at St Andrews following his £1.6million move from Bolton Wanderers in August 2006.
He played a major role in helping the club regain their Premier League status last season and it came as a big surprise to many fans when he was released by the club in the summer.Debut Apps (+ sub) Goals Yellows Reds 19 Aug 2006 93 (0) 6 18 2 30 May 2006 4 (0) 1 2 0 25 Aug 2004 43 (9) 8 4 0 07 Apr 2000 15 (0) 1 2 0 First App. Apps (+ sub) Goals Yellows Reds 27 Sep 2008 31 (0) 0 6 0 18 Aug 2007 20 (0) 0 6 0 19 Aug 2006 42 (0) 6 6 2 13 Aug 2005 27 (1) 4 3 0 25 Aug 2004 20 (8) 5 3 0 01 Jul 2001 8 (0) 0 0 0 08 Jul 2000 4 (0) 0 1 0 07 Apr 2000 3 (0) 1 1 0