The 20-year-old left Old Trafford last year in search of more first-team opportunities but has endured a largely disappointing spell in Germany, where he has started only three matches.
He has made 12 appearances for Dortmund in total and has not started in the league. His last game for Dortmund was their Europa League defeat by PAOK Salonika on Dec. 10.
"Adnan will rejoin the first-team squad immediately," United said on their website (www.manutd.com).
Januzaj, who is contracted to United until 2018, made 22 appearances under Louis van Gaal in 2014-15 and scored the winner against Aston Villa in a Premier League match this season before moving to Dortmund.
He emerged under then manager David Moyes in 2013, scoring two goals to secure a 2-1 win for United at Sunderland in his first start for the Premier League club.
United are fifth in the Premier League table, with Van Gaal under increasing pressure to ensure a top-four finish after a series of disappointing performances.