Muzzy Izzet has been forced to retire after failing to overcome a long-standing knee injury.
The midfielder missed the majority of Birmingham's relegation season last term due to the problem and it has shown little improvement of late.
Blues chief Steve Bruce said on Monday that Izzet was in real pain when putting any stress on the joint and the 31-year-old has taken the decision to call time on his career.
"Muzzy has come to the point where his knee is very, very painful and he can't fulfil what he wants to do," Bruce told the club's official website.
"We would like to thank Muzzy for his efforts during his two years here and wish him all the very best for whatever he chooses to do in the future."
Izzet, a former Turkey international, had a further year to run on his contract at St Andrews, but he agreed to cancel the deal by mutual consent.
The midfielder started his career at Chelsea, but made his name at Leicester during an eight-year stint with The Foxes.
He joined Blues in the summer of 2004, but was never really able to show his class at St Andrews.