The 18-year old starlet made his first start for United at Sunderland in October 2013 and scored twice in a 2-1 win at the Stadium of Light.
United 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." The Scot continued, "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 himself also spoke about his decision to sign an extension, "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. 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."
The Belgian-born midfielder was signed by United at the age of 16 from Anderlecht, and is eligible to play for his country of birth at international level, as well as Albania and Turkey. England boss Roy Hodgson spoke of his admiration earlier in the week by saying if there was a way he could get Januzaj to play for England, he would certainly look at the angles.
The Red Devils sit in 9th position in the English Premier League going into today's round of games with 10 points from seven games, and entertain Southampton at Old Trafford looking to move further up the table. Robin van Persie and Tom Cleverley are doubts for the game, while Rio Ferdinand could return to the heart of the defence.