The versatile defender-cum-midfielder, 25, arrives at St Andrew's having been released by fellow relegated side West Ham at the end of last season.
New addition: Birmingham City confirmed the signing of Jonathan Spector on a free transfer
Spector, a USA international who has spent the past five years at Upton Park following a move from Manchester United, underwent a medical at Blues on Monday before putting the finishing touches to a two-year contract, adding much-needed depth to Chris Hughton's new-look side in the npower Championship.
Hughton expressed his delight to have secured Spector's services, as he told the club's official website, bcfc.com: 'Jonathan is a player who has got experience of playing in the Premier League and as an international for America so he has good credentials. And at 25 he's at a very good age.
'He is versatile and can play in a few positions. I think he will be a big asset for this club.'
Versatile: Spector played at full-back and in midfield for West Ham last season
Birmingham, who are working under tough financial constraints following May's relegation from the top flight, have already captured Chris Burke, Marlon King, Morgaro Gomis, Adam Rooney and Steven Caldwell all on Bosman free deals. Goalkeeper Boaz Myhill has also joined on a season-long loan.
And Hughton admitted that he is looking to add more players to his squad.
He added: 'There is still some work to do on the transfer front because there's a long season ahead and a lot of games to play and we need to have a squad in place that will be competitive in the npower Championship.
Looking to add: Hughton hopes Spector won't be the last player through the door at Birmingham
'Jonathan is another addition that helps us get closer to achieving exactly that.'
West Brom's Myhill was part of the deal which saw Ben Foster head in the opposite direction to The Hawthorns on a similar arrangement, while the likes of Craig Gardner, Roger Johnson, Sebastian Larsson, Barry Ferguson, Lee Bowyer and Kevin Phillips have also departed.
There are likely to be further casualties before the close of the transfer window on August 31, with prized centre-back Scott Dann, fellow defender Liam Ridgewell and strikers Cameron Jerome and Nikola Zigic all linked with moves away from the midlands club.
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