The 26-year-old winger was loaned to Rangers at the start of this season but Fulham boss Roy Hodgson has confirmed that Cook has made a permanent move back to Loftus Road for an undisclosed fee.
"Lee, as far as I understand, has been transferred permanently to QPR," said Hodgson.
Cook joined Fulham from Rangers in 2007 but never made a single first team appearance during his time at Craven Cottage.