The defender spent the previous two campaigns on loan at Rayo Vallecano and Malaga, enjoying life as a first-team fixture at both.
That was a far cry from his experience at Santiago Bernabeu, where he made just two substitute appearances in the league since 2012.
"Everything I have heard about San Sebastian has been very good," he said. "I'm very happy that a club like Real Sociedad have shown an interest in me.
"Real Sociedad has a very good reputation - both in Spain and in Europe."