The 20-year-old, who is currently on loan at Gillingham, will remain at Loftus Road until 2015.
Harriman 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."
Harriman has made 11 appearances for Gillingham during his loan spell with the League One side.