United wished the 22-year-old winger well on Twitter:
Manchester UnitedGood luck to @AdnanJanuzaj, who has left #MUFC to join Real Sociedad. https://t.co/K2SSIXj9e8
Januzaj joined United's youth academy from Anderlecht in 2011 and impressed as a teenager in his first senior appearances for the Red Devils under manager David Moyes in the 2013-14 season.
However, his progress has stalled in the years since. He endured a miserable loan spell at Borussia Dortmund in 2015-16, which was ended early due to a lack of game time.
In 2016-17, he spent a season on loan at Sunderland, making 25 Premier League appearances and returning three assists as the Black Cats were relegated.