The reigning Ligue 1 champions have inherited a talented English midfielder, one whose inner demons are regularly unleashed both on and off the field.
In no particular order, here are 10 key moments from Joey's lowlight reel...
On an evening in December 2007 in which he consumed "10 pints of lager and 10 bottles of lager" (a questionable indulgence for any professional footballer), Barton viciously attacked a teenager outside a McDonalds in Liverpool, punching him 20 times and breaking his teeth.
He received a six-month sentence for assault and affray, and a stint in rehab.
With his club teetering on the edge to relegation against a team looking to secure the Premier League title on the final day of the 2011-12 season, Barton lost his temper with Carlos Tevez (a man he has since dismissed as a mercenary). He elbowed the striker in the face and received a red card, and then proceeded to attack Sergio Aguero, headbutt Vincent Kompany and exchange angry words with Mario Balotelli on his way off the field.
In addition to the 12-match ban that ended his QPR career, he received a £75,000 fine.
During a December 2010 match with Blackburn at St James' Park, Barton let his fists do the talking once again when he punched Mort Gamst Pedersen in the chest.
The referee failed to spot the incident at the time, but the FA later charged him for violent conduct and banned him for three games.
Man City's July 2004 game with Doncaster may have been a friendly, but there was nothing friendly about the 10-man brawl that Barton started after an unnecessarily aggressive challenge on Rovers' Paul Green.
He was only given a yellow card and a dressing down from manager Kevin Keegan, but had this been a competitive fixture he would almost certainly have seen red.
In May 2007, Barton felt compelled to beat colleague Ousmane Dabo so hard during training that the Frenchman was left unconscious and had to be shipped off to a hospital with a detached retina.
Barton was arrested, and after eventually pleading guilty to assault, was ordered to undertake 200 hours of community service and pay Dabo's court costs. The FA gave him another violent conduct charge and a six-game ban, while the incident effectively ended his Manchester City career.
Barton was born and raised in Everton, but he received copious amounts of abuse from fans during a game at Goodison Park in 2006. When Man City equalised in injury time, he dropped his shorts and proceeded to moon the home crowd.
Although it was one of the more lighthearted incidents in his canon of misconduct, he was fined £2,000 by the FA for bringing the game into disrepute. How cheeky.
For most, Christmas is a time for giving and compassion. Barton, however, did not get into the holiday spirit in 2004 when he stubbed out a cigar in the eye of youth player Jamie Tandy (pictured) at the Man City Christmas party, nearly blinding him.
On his official website, Barton insists he was only trying to stub out the cigar on Tandy's neck. Regardless of his bad aim, he received a fine of six-weeks' worth of wages, while Tandy was shipped off on loan to Scandinavia.
Barton clearly lost the temper that he had claimed he had tamed, and when the Arsenal debutant raised his hands in the conflict, he was sent off, while the Newcastle man was cautioned with a yellow.
Barton joined QPR a week later.
During a 2005 preseason tour of Thailand, Barton was provoked by a 15-year-old Everton fan who had kicked his shin. Barton claims he merely "slapped" the youngster in return, but reports suggest he assaulted him and needed to be restrained by a teammate.
He was sent home from the tour, made to pay £120,000 in fines and ordered to attend an anger management course.
Barton voices his oft-controversial brand of opinion through his Twitter feed on a regular basis. On one occasion in 2011, he said, "If only we as players could tell the fans exactly how it is, without them above fining us lots of money." Shortly after, he did exactly that, heavily criticising his employer, Newcastle.
The next day he was made available on a free transfer.