With his current deal set to expire at the end of the season, the 18-year-old’s future had been the source of much speculation since he scored twice on his full Premier League debut against Sunderland two weeks ago.
However, United revealed on their website that Januzaj had agreed a new deal to take him through to June 2018.
Of the new contract, manager David Moyes said: "I'm delighted that Adnan has committed his future to United. He is an outstanding talent and has impressed me consistently since I came to United in July.
"His balance, poise and ability to beat players is truly exceptional for a boy of his age. It’s great that he recognises that this club, with its tradition for developing young players and giving them a chance on the biggest stage, is the best place for him to be."
It has been a meteoric rise for the Belgian-born 18-year-old, who only made his debut in the Community Shield against Wigan in August, and he also spoke to the official club website.
"It’s a great thrill to be able to sign for Manchester United. Since arriving here I have always felt like this was the right club for me."
"It’s like a dream. I want to work hard now and establish myself as a Manchester United player."
The club's twitter account tweeted a picture of the player putting pen to paper: