Luton have made their ninth summer signing following the arrival of winger Danny Green.
Notts County have signed Togo international Mawouna Amevor on a two-year deal.
Barton is currently on loan with the Ligue 1 club, who took him on in August while he served a 12-match ban incurred during the last game of the 2011/12 season against Manchester City.
The midfielder, who is a frequent and controversial user of Twitter, used the social networking site to reveal that he would not be going back to Loftus Road regardless of their league status and that OM are now "in his blood".
Over a series of Tweets he insisted he held no grudges against QPR, made a pointed jibe at former boss Mark Hughes and referenced an apparent deal between Rangers and Marseille for his services.
"Some strange people think that I'll be playing in the Championship next season?? Good one! QPR might, I won't! #fact," he wrote.
"It's not up to QPR if I stay. Agreement already in place for me to stay here. Can't see QPR wanting me on the wage bill in the Champ.
"My QPR career was over when they decided to listen to the footballing sage, Mark of Hughes...
"I'm loving life in France. Loving Marseille. They love me. All I want to do is give everything for this football club. They backed me when nobody else did, for that I will be eternally grateful. This club is now in my blood, its impossible to get it out (sic) #ForzaOM.
"I really hope they (QPR) stay up. I have all season. Stay up/go down. Harry stays/goes. I don't want to be part of it. Marseille is my home now."