The 25-year-old, who can play in defence or midfield, was a free agent after his contract with West Ham United - who were relegated like Blues from the Premier League in May - expired at the end of last season.
"Jonathan is a player who has got experience of playing in the Premier League and as an international for America so he has good credential. And at 25 he's at a very good age," Blues boss Chris Hughton told the club's official website.
"He is versatile and can play in a few positions. I think he will be a big asset for this club."
Spector, who has signed a two-year contract, moved to England in 2003, playing three times for Manchester United. He then moved to West Ham in 2006. He made his international debut in 2004 but was restricted to the bench at last year's World Cup in South Africa,
Birmingham were relegated at the end of last season but also enjoyed the high of beating Arsenal in the League Cup final -- their first top level silverware since winning the same trophy in 1963 -- which books them a place in this season's Europa League.
Source: AFP
Source: AFP