Barton was placed on the transfer list at St James' Park, with the club prepared to let the 28-year-old leave on a free after he voiced criticisms on Twitter.
The midfielder had a medical with the Loftus Road outfit this week and said he would decide on Friday whether to stay at St James' Park or move to QPR.
Barton told QPR's official website: "It's been quite a whirlwind, so to get things finally sorted and draw a line under it all is a great relief.
"The ambition of the club really impressed me.
"There's obviously a great set of lads here already and that's why they had such a great season in the Championship.
"The gaffer is talking about adding two or three more bodies and that, combined with the potential of the club and the new owners' ambitions, bodes really well for the future."