The Tunisia defender, 30, has signed a three-year contract with the Championship side.
Birmingham manager Steve Bruce said, "It is a great signing for us, as Radhi is a vastly experienced centre-half, he has got two seasons' worth of experience of English football and performed extremely well at the highest level.
"With Matthew Upson's injury, it gives us more cover in that position and will strengthen our squad for the campaign ahead.
"We are absolutely delighted to have secured his services for three years and we are still looking to add more players to our squad as well."
Meanwhile, Lorenzo Stovini could be set to join Bolton Wanderers after the player's agent, Giocondo Martorelli, revealed the defender will travel to England to link up with Sam Allardyce's squad on Tuesday.
Martorelli told skysports.com, "Lorenzo Stovini will be in England probably on Tuesday evening."
"It's not a trial. There is already an agreement with the club, for three years. He will undergo medical checks.
"The guy wants to understand what type of environment there is and what type of impact he will have, and the club want to see him in action too.
"It's a reciprocal thing. I think about three days will be enough. They want to see if he's in good condition, Stovini wants to see what the environment is like.
"For this reason, I decided to not go with him. I know Bolton are hoping to take him to Valencia for their Spanish tour."
"The negotiations have developed in the last 20 days. I don't know how many times Bolton have looked for him," Martorelli added.
In other news, Polish trio Jaroslaw Fojut, Blazej Augustyn and Prezemyslaw Kazimierczak have all put pen-to-paper on their first professional terms with the club.