The 18-year-old was due to be out of contract next summer, but, following his match-winning double against Sunderland, the Red Devils have moved swiftly to secure his long-term future.
As revealed by Goal, the Premier League champions, chastened by the departure of Paul Pogba, had ramped up their efforts to keep Januzaj at the club in recents weeks and have now rewarded him with a bumper new deal.
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"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," Januzaj told the club's official website.
"This season has been great for me – going on the pre-season tour, making my debut in the Community Shield win at Wembley and then breaking into the team at Sunderland – it’s like a dream.
"I want to work hard now and establish myself as a Manchester United player."
United boss David Moyes is thrilled Januzaj has signed a new deal, ending the speculation surrounding the youngster's future, and reiterated his belief that Old Trafford is the perfect place to continue his development.
"I'm delighted that Adnan has committed his future to United," said the Scot. "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."