Cook joined Fulham for £2.5million in July 2007 but was allowed to join Charlton on loan in January and spent the second half of the campaign with the Addicks.
Fulham boss Roy Hodgson is rebuilding his squad after the Cottagers escaped relegation from the Barclays Premier League, and he has decided Cook does not form part of his plans for the coming campaign.
Cook is relishing his return to QPR, where he was a hugely popular player until his departure last summer.
"I'm absolutely delighted to be back," he told qpr.co.uk.
"Once it became clear I could leave Fulham there was only one club I wanted to move to."
Dowie said of his new signing: "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."