The 28-year-old was in the final year of his contract at St James' Park and made available on a free transfer recently after criticising Newcastle's recruitment policy via his Twitter account.
Barton passed a medical at QPR's training ground in Harlington on Thursday but left without penning a deal. Having mulled over the move overnight, Barton has now penned a deal with the west London club and could make his debut against Wigan on Saturday or at home to Newcastle after the international break.
"It's been quite a whirlwind, so to get things finally sorted and draw a line under it all is a great relief," he told the club's official website, www.qpr.co.uk.
"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."
Barton becomes the first signing at QPR since AirAsia boss Tony Fernandes completed his takeover of the club last week.
Rangers boss Neil Warnock has since had little time to make his mark on the transfer market but is delighted to get the first of the five new faces he hopes to bring to Loftus Road before Wednesday's deadline.
"We've got a very, very good player on our hands," he said of Barton, who will wear the number 17 jersey. "He's coming here to show people what he can do. He wants to get back into the England team.
"He's very enthusiastic. People will say one or two things about him, but they've said stuff about me in the past too, so I won't worry about that."
He added: "He's a winner. Even without the armband on, he's a captain every week."
Source: PA
Source: PA