The 26-year-old returns to La Liga on a season-long deal with an option to make the move permanent at the end of the campaign, rather than stay with QPR in the Championship
The move marks a return to Spain for the 26-year-old, who joined the English club from Real Madrid in 2012.
|50/1||QPR are 50/1 with William Hill to be relegated from the Championship
"Esteban Granero has returned to Spain to join Real Sociedad on loan," read a statement on QPR's official website on Thursday.
Fulham had expressed interest in his services and were reported to have made an enquiry for the former Spain Under-21s international but he has elected to return to La Liga.
Real Sociedad have confirmed that the midfielder will arrive in San Sebastian on Friday and that they have the option to make the deal permanent at the end of the season.
La Real have been in the market for midfield reinforcement ever since Asier Illarramendi joined Real Madrid for €30 million (£25.6m) in July.