The midfielder, who left Norwich at the end of last season, is familiar to Lee Clark from the City manager's time as a coach at Carrow Road.
Clark told Birmingham's official website: "I worked with him during my time at Norwich when he was a youngster at the club.
"I've followed his progress since, He's done exceptionally well on loan at a few different clubs and I'm delighted to bring him here.
"Tom is a terrific young lad who is a powerful and athletic midfield player who can get forward and score goals."
Adeyemi, who spent last season on loan at League One Brentford, will officially become a Birmingham player on July 1.