The academy graduate, who has made just one league appearance for the Hoops, will remain at Loftus Road until 2015.
Harriman is currently on loan at Sky Bet League One outfit Gillingham and has played 11 matches for the Gills this campaign.
He told QPR's official website: "It's nice to secure my future here for another year.
"It gives me the confidence to go out and enjoy my football, not worrying about stuff behind the scenes.
"Hopefully after my loan spell I can return a better player and push on at QPR."