The 26-year-old moved to Loftus Road from Real Madrid last August in a reported £9m deal but could not help Harry Redknapp's men avoid relegation.
A statement from QPR read: "Esteban Granero has returned to Spain to join Real Sociedad on loan.
"The 26-year-old midfielder has penned a season-long loan deal with the La Liga outfit.
"Granero, of course, joined QPR from Real Madrid last summer and has since made 28 appearances in all competitions for the Rs, scoring one goal.
"The deal is subject to international clearance."
The arrangement also offers Sociedad the option to purchase the player at the end of the season.
Sociedad confirmed Granero had passed a medical in London this afternoon and will travel to San Sebastian tomorrow ahead of his official presentation.