Chris Smalling has agreed terms and passed a medical examination to join Manchester United in July 2010 on a four-year contract for an undisclosed fee.
An England Under-21 international, Smalling has made 10 first-team appearances for Fulham.
Sir Alex Ferguson said: "Chris is an extremely talented young defender, who we have been monitoring for some months. He’s very quick and reads the game well. He will be a great asset to the club, playing alongside some of the best defenders around and we’re delighted to have secured his signature."
Chris Smalling said:
"My time at Fulham has been great and I have learned a lot about the game from Roy Hodgson and the coaching staff.
“But the chance of coming to the biggest club in the world is something I just couldn’t turn down. I’m excited about working with Sir Alex and the fantastic players in the squad.
"I’ve played a few times in the Europa League this year, but the thought of making the step up to the Champions League next year is great. I can’t wait.”