The 26-year-old has returned to La Liga on a season-long deal with an option to make the move permanent at the end of the campaign
The move marks a return to Spain for the 26-year-old who joined the English club from Real Madrid in 2012.
The midfielder has been heavily linked with a move away from the club since its relegation from the Premier League last season, and he has now penned a deal with the Champions League newcomers.
"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 player this week, but the former Spain U21 international has elected to return to La Liga.
Real Sociedad has confirmed that the midfielder will arrive in San Sebastian on Friday and that the club has 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 euros in July.