The Cottagers have confirmed the arrival of the Morocco international, who has revealed that he wants to return to Loftus Road "one day".
"This was a difficult decision for me, but at this stage of my career I feel I need to be playing at the top level," BBC Sport quotes Taarabt as saying.
"I hope the fans understand my decision and I really hope to return to Loftus Road one day. I wish QPR well for the season and hope they achieve promotion back to where I believe the club belongs."
The 24-year-old made over 150 appearances for QPR.