United confirmed the loan on their official website and he will join Donald Love and Paddy McNair who joined earlier in the week.
The Black Eagles were closely linked with a move for Januzaj as they continue efforts to find a replacement for Gokhan Tore who joined West Ham on a one-year loan over the summer.
Januzaj is no stranger to David Moyes who was in charge of United back in 2013. In fact the Belgium international scored a brace against Sunderland in October 2013 on his full debut to earn the Red Devils a 2-1 victory.
The 21-year-old is expected to be included in the Black Cats squad ahead of their clash against Manchester City on the weekend.
Januzaj has failed to live up to his expectations over the past few seasons but he was once highly rated, in fact Sir Alex Ferguson himself singled him out during his time in charge of the club.
“Adnan is a beautifully balanced player,” Ferguson was quoted as saying by the Telegraph.
“He’s only 18 and has to grow into his frame but he has good balance, good acceleration and is a very good technical player.”
Januzaj will embark on his second loan deal after joining Bundesliga giants Borussia Dortmund last season.