The 18-year-old's previous deal was due to expire in the summer, which in turn had seen him linked with a host of clubs, including Juventus, Barcelona and Manchester City.
However, the Belgium-born attacking midfielder has now ended that speculation by signing on at Old Trafford for five more seasons.
"It's a great thrill to be able to sign for Manchester United," he told the club's official website. "Since arriving here I have always felt like this was the right club for me.
"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."
Moyes on Januzaj deal: "Adnan's an outstanding talent. His balance, poise & ability to beat players is exceptional." pic.twitter.com/dGwPQejwnj— Manchester United (@ManUtd) October 19, 2013
Meanwhile, manager David Moyes added: "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."
Januzaj joined United as a trainee from Anderlecht in 2011.