The England left back, who featured for the Three Lions at the World Cup, completed his medical at the club's training ground Friday, having agreed to terms on a lucrative contract.
Shaw will earn around 160,000 pounds a week at Old Trafford, making him the highest-paid teenager in soccer.
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Upon completing his move, he told the club's website: "I am thrilled and excited to be joining Manchester United. I have been at Southampton since I was 8 years old and I would like to thank them and the fans for everything they have done for me.
"I want to continue to progress my career and joining United is the ideal place for me to do that. I am looking forward to this new chapter in my life and also looking forward to learning from the world-class players and management at the club."
The 19-year-old becomes the second signing of Louis van Gaal's United reign, with Ander Herrera having completed his move to the club Thursday.