The 20-year-old left Old Trafford last year in a bid to get more regular first-team opportunities but he has started only three matches for the Bundesliga side and United have decided to bring him back to England.
Januzaj sprung to prominence in 2013 when he scored two fine goals to secure a 2-1 win for United at Sunderland in his first start for the Premier League club under then manager David Moyes.
He made 22 appearances under new manager Louis van Gaal in 2014-15 and scored the winner against Aston Villa in a Premier League match this season before moving to Dortmund.
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United are fifth in the Premier League, nine points behind leaders Arsenal after 20 matches, and Van Gaal is under pressure to qualify for next season's Champions League by finishing in the top four.
(Reporting by Ed Osmond; editing by Toby Davis)
