The former Newcastle and Manchester City midfielder sealed his loan move to the former European champions on deadline day, after two days of negotiations, with Cameroon international Stephane Mbia moving in the other direction.
Barton was presented to the media at Stade Velodrome, ahead of the Ligue 1 leaders' encounter against Rennes later on Sunday.
The 30-year-old had earlier told Telefoot that playing for Marseille represents the biggest challenge of his career.
"I never imagined [that I would] one day play in France. This is the biggest challenge of my career. I need to be in good physical condition. I don't think I will be at the top until after three or four weeks of intensive training.
"I'm very excited to wear this jersey and play for Marseille. I immediately accepted this opportunity. OM may well be part of the English league - football and the city are intrinsically linked, which is not the case in London or Paris," the one-time England international later said in his presentation press conference.
"I remember when I was 10 or 11 years old and the OM of [Didier] Deschamps and [Basile] Boli [and Alen] Boksic won the Champions League [in 1993]."
Speaking about his remaining nine-match suspension from the FA, incurred against Manchester City on the last day of the 2011-12 Premier League season, Barton remains uncertain about whether the ban will be transferred to his new club.
"We do not know yet what will happen. I must repair the damage I have caused. I was stupid on several occasions in England. I want to show my qualities. But I'm a good guy.
"Those who know me know that I'm not a bad person. I hope they will judge me correctly. For me, the important thing is to play and win. OM is perfect, especially in terms of mentality. It is the same passion [as] in England.
Marseille aim to continue their 100 per cent record against Rennes later on Sunday. Kick-off is at 17:00CET.