The Spaniard moved to Loftus Road in 2012 but suffered relegation from the Premier League in his debut season.
Former Real Madrid man Granero was then loaned to Sociedad but featured only seven times in an injury-hit season where he tore the anterior cruciate ligament in his right knee.
Having secured a league winners medal at the Santiago Bernabeu in his three-year spell with the Spanish giants, Granero will return to La Liga next term with Sociedad.
"[Chief executive] Philip Beard did the deal at the weekend," the 67-year-old said on Monday.
"I think Esteban is going back and I wish him well. He's a great lad – a fantastic boy."
The 27-year-old has made 28 appearances for QPR in all competitions, scoring one goal.
QPR later confirmed that Grenaero had been sold to the Basque club for an undisclsoed fee.