The 20-year-old forward underwent a medical at Old Trafford on Monday afternoon, 24 hours after his two-year contract at Sunderland had officially ended, and signed a four-year deal with the Premiership title holders.
"I am really excited to be joining Manchester United," he told the Utd official website. "When you are young they are one of the clubs you dream of playing for.
"I am ready to work hard and give 100% to United, and I'm already focused on the new season ahead."