The 27-year-old has left Loftus Road after two years and returned to Spain following a loan spell with the Liga side last season
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.
The 27-year-old has made 28 appearances for QPR in all competitions, scoring one goal.