Mbia, 26, who can also play in midfield, is QPR's 12th signing since the end of last season. He has agreed a two-year deal.
Mbia, capped 36 times by Cameroon, played 103 games for Marseille in three years at the club.
"Stephane is a very good player and can play in a number of positions," Rangers manager Mark Hughes told the club's website (www.qpr.co.uk).
"He is very adept as a defensive midfielder but he has been regularly playing at centre back so we have got another player who can add to the flexibility of the squad.
"He is a big, strong guy with a strong mentality. He wants to win and is the type of player I want at the club."
Mbia said he was delighted with the transfer.
"It was an easy decision because this is an important move for my career," he explained. "It has always been a dream of mine to play in the Premier League which I believe is the best league in the world.
"The ambitions of the club and what the board of directors and the manager want to achieve here excites me greatly."
Barton will spend the season on loan in the south of France, although it is unlikely he will be returning to QPR when that deal expires.
Barton has been frozen out of QPR's plans after his sending-off against Manchester City in the final game of last season resulted in a 12-match ban.