The 27-year-old Cameroon ace has moved to Spain on a season-long loan to press his international claims in a World Cup year.
He arrived at Loftus Road from Marseille last summer for an undisclosed fee and went on to make 32 appearances last season as the club were relegated from the Premier League.
Looking forward to his new challenge, Mbia said: "This is a brilliant club and I am looking forward to having a great year and helping Cameroon reach the World Cup.
"To do that I need to be playing regularly in a top division against the best players in the world.
"I would like to thank QPR for giving me an opportunity to play in La Primera Liga."