Forest have lost Chris Cohen to a cruciate knee injury so have been on the lookout for cover and Hill, who can operate as a central defender or at left-back, has now joined for the maximum 93-day period.
Forest manager Steve McClaren said: "It's a bitter blow for Chris, who has been one of our stand-out performers so far this season.
"But we're delighted to have agreed terms with QPR for Clint. He has great experience and will add to our defensive options."
He is expected to make his debut for Forest at Watford on Saturday.