It marks a return to his homeland for the 26-year-old, who left Spanish football in August 2012 as he opted to try his luck in the Premier League after signing a four-year contract at Loftus Road.
However, the London club's time in the top flight was cut short following relegation in April and Granero has followed the likes of Loic Remy in departing.
After impressing during his debut campaign with QPR, in which he made 28 appearances in total, Granero is back in Spain with Sociedad.
The move also appears to signal an apparent snub for Fulham, who are reported to have made an enquiry about the former Spain Under-21 international's availability shortly before the loan deal with Sociedad was agreed.
Granero will now be hoping to be involved when Sociedad start the La Liga season with a home match against Getafe on Saturday.
