QPR boss Iain Dowie said: "Lee is a class act and will add to the already healthy options we've got in the attacking third.
"He's proven at this level, has great quality and he'll certainly have some fire in his belly after a very frustrating season with Fulham."
QPR sporting director Gianni Paladini had been trying to secure Cook's return for several months.
Successive attempts to agree a deal with Fulham failed and Charlton pounced to sign Cook on loan before the transfer window closed in January.
Rangers remained desperate to capture him but wanted an initial loan deal, allowing Cook to prove his long-term fitness before a transfer fee, believed to be around �2.5m, was paid to Fulham.
"This is a truly special day for the football club," said Paladini.
"I'm delighted to have him back here. We're really excited about his return and can't wait to see him back in the famous blue and white hoops."
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