Januzaj will join up with the first team squad immediately, although is unlikely to be involved in Saturday's Premier League opener against Manchester City, despite being available for the clash.
Lack of progression following life under Moyes
He'll be looking to make an immediate impact at the Stadium of Light, having suffered a difficult couple of years under former United boss Louis Van Gaal, having burst onto the scene under now-Sunderland gaffer David Moyes in the 2013-14 season.
During one of his first senior appearances for the Red Devils, Januzaj bagged a brace at the Stadium of Light to hand United a 2-1 win against Sunderland, giving fans much optimism about his future.
Unfortunately for the Belgian, he didn't go on as planned from there, finding opportunities limited under Van Gaal the year after, before being loaned out to Borussia Dortmund last season. His time in Germany was again frustrating, struggling to break into the team.
Considering all this, he'll be pleased to link up with the man that gave him his chance, Moyes, as he joins Sunderland's yearly battle to beat the drop.
Much change at the Stadium of Light
It's some good news for Sunderland in a week where it became clear that they're likely to lose defender Lamine Kone to Everton, seemingly replacing him on Thursday, confirming the signatures of Paddy McNair and Donald Love.
They'll be hoping that the defensive saga, plus trading manager Sam Allardyce for Moyes a matter of weeks ago, won't impact them as they start their season on Saturday, with kick-off at the Etihad Stadium set for 5;30PM.