The 31-year-old, who has just returned from World Cup duty in Japan & South Korea has Premiership experience and will add strength-in-depth to the Blues back-line.
Cunningham told the club's official website: "I am very pleased to be signing for Birmingham because it is a very unexpected opportunity for me."
"After the World Cup I was looking forward to starting the season with Wimbledon but things happen very quickly in football."
"I was made aware of Birmingham's interest and after speaking with Steve Bruce on a number of occasions, thankfully the deal was concluded."
"It was an opportunity too good to turn down."
Cunningham went on to explain his lack of pre-season training, "I haven't done any training yet and obviously the lads here have been back for a week-and-half."
"I will travel up to Scotland for the tour and may play some part in the games up there."
The Don's skipper and player of the season 2001/02 made over 300 appearances in his 8 years at the Milton Keynes bound club.