"Real Sociedad have reached an agreement with David Moyes to be the club's first-team coach until June 30, 2016," read a short club statement.
Moyes had been without a job since his turbulent 10-month spell at United ended in April. The Scot previously spent a productive 11 years at Everton.
Real, who beat beat champions Atletico Madrid 2-1 on Sunday, are 15th in La Liga, with just nine points from their opening 11 matches.