Thirty-year-old Spector was Blues' longest-serving senior player, having been at the Club since August 2011.
His existing deal was due to expire in the summer, but Blues boss Gianfranco Zola was happy to let the American international return to his home country early, with the MLS season due to start in March.
Spector's contract has been cancelled by mutual consent and a replacement has already been brought in at right-back, with Emilio Nsue joining from Middlesbrough last week.
During his time at St. Andrew's, Spector played 179 times for the First Team and netted just a solitary goal.
Born in Arlington Heights, a suburb of Chicago, Spector began his youth career with local side Chicago Fire, but was snapped up as a teenager by Manchester United after being spotted playing for the United States Under-17s team as a striker.
He spent three years at Old Trafford, before moving to West Ham United where he was based for five seasons, prior to making the switch to Blues.
Speaking last week, Blues boss Zola commented: “I am a little bit disappointed and sad that Jonathan Spector will be leaving us. He deserves that. He’s going to go home and I wish him all the best.”
Everyone at Birmingham City would like to wish 'Specs' good luck for the future and thank him for his service to the Club.