The 22-year-old has agreed a five-year deal to play at the Anoeta Stadium after being left out of the Manchester United squad to travel to the US, reports Xinhua news agency.
Januzai broke into the Manchester United first team in the 2013-14 season under David Moyes, who later tried to sign him when he was in charge at Real Sociedad, but then started getting fewer opportuninies.
He spent the last two seasons on loan at Borussia Dortmund and Sunderland but with only limited success.
The Belgian expressed his happiness over joining Real Sociedad, saying he aimed to help the side from the city of San Sebastian win either the Europa League or the King's Cup knockout tournament in the coming season.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
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