The 26-year-old midfielder left Real Madrid for west London last August in a reported £9million deal but could not help Harry Redknapp's men avoid relegation from the Premier League at the end of the 2012/13 season.
A QPR statement 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 from QPR at the end of the season.
The Basque outfit confirmed Granero had passed a medical in London on Thusday afternoon and will travel to San Sebastian on Friday ahead of his official presentation.
Sociedad face Lyon in the Champions League play-offs after finishing fourth in the Primera Division last season.
Former Spain Under-21 international Granero, who can play in the centre of midfield or in a more advance role, will bolster coach Jagoba Arrasate's options after Asier Illarramendi joined Real Madrid for 38.9million euros last month.