Hargreaves, who has 42 caps for England, joined the Rs squad for his first session today, the London club confirmed.
The 31-year-old moved to Manchester United from Bayern for a fee believed to be around £17 million in 2007 but he has been plagued by injuries over the last four seasons.
Hoops boss Mark Hughes told www.qpr.co.uk: "Owen is training with the group. There is no obligation for either party.
"He is keen to retain his fitness and we are more than happy to help him by allowing him to train with us and use our facility at Harlington."
Hargreaves' struggles in the last few years prompted him to contemplate retirement after leaving City but by training with QPR, it seems he is determined to continue his playing career.
It appeared he had won his fitness battle when he made a goalscoring Blues debut in a Carling Cup encounter against Birmingham in September.
However, Hargreaves made just three more appearances for City and was not seen again after being introduced as a late substitute in the FA Cup third-round defeat to United at the Etihad Stadium in January.
And only last month he said: "I don't want to chase something. If I get the right fit I will go for it.
"But I am not forced to do anything and if I can't get back to the level I used to, I don't want to play."