The former England international has signed a one-year contract with the Premier League giants.
Hargreaves joined City on a free transfer after having been released by United when his contract expired at the end of last season.
Hargreaves struggled with injuries during his time at Old Trafford, but has now been given a chance to prove himself at Eastlands.
A statement on City’s official website read: “The England midfielder joins as a free agent having left Manchester United at the end of last season. He has signed a one-year contract.”