Hargreaves, who has 42 caps for England, joined the Rs squad for his first session on Saturday, the London club confirmed.
The 31-year-old moved to Manchester United from Bayern for a fee believed to be around $25 million in 2007 but he has been plagued by injuries over the last four seasons. He spent last season with City after being given a free transfer by Alex Ferguson.
English Premier League
Brazilian midfielder Fernandinho has confirmed he wants to join Manchester City.