Barton flew by private jet with his management to the southern port city in France to discuss terms with Marseille officials including sporting director Jose Anigo.
Barton is expected to agree a loan deal with Marseille after he signed a lucrative four-year contract following QPR's promotion to the Premier League last year.
"Just finished the 1st part of my medical at Marseille. It's great to finally be here. Back tomo to finish it off," the 29-year-old tweeted.
Barton has been frozen out of QPR manager Mark Hughes's plans after his sending-off against Manchester City in the final game of last season resulted in a 12-match ban.
The former Newcastle and City player had this week vented his frustration on Twitter, saying he had been forced to train away from the first team and revealed to his followers that he wanted to leave the club.
He also Tweeted on Tuesday that the deal with Marseille had fallen through.
"The best scenario for me personally and I think it for QPR and probably for Mark Hughes is that I'm sort of out of his hair for a little bit," Barton told Sky Sports before his flight to France.
"It's not a great position for me to be in as a professional, when you feel unwanted and unloved, and you haven't got a shirt number or a locker at the training ground."
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