Smalling will join United in July 2010 on a four-year contract for and undisclosed fee.
Reds boss Sir Alex Ferguson told the club’s official website: “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.”