Furlong’s contract was due to expire next year and he has been given two-year extension, keeping him at Loftus Road until 2021.
The 22-year-old, a product of Rangers’ youth system, has made 39 league appearances for the R’s.
He was recently ruled out for at least three months with a knee injury.
“Darnell fully deserves this new deal and I am delighted for him,” manager Steve McClaren told the club website.
“He is a huge asset to the club and when he is back and available it will feel like a new signing for us.”
Meanwhile, QPR have rejected an approach from Middlesbrough for Luke Freeman.