The 21-year-old former Maidstone stopper arrived at Old Trafford last July having agreed to sign for Sir Alex Ferguson's side from Fulham six months previously.
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And the youngster - who cost the Red Devils £10 million - has enjoyed a successful first season in United's colours, making 33 appearances in all competitions as deputy to established centre-backs Rio Ferdinand and Nemanja Vidic.
Smalling, who has won 14 caps for England Under-21s and played alongside new United recruit Phil Jones at the recent European Championships in Denmark, is expected to see more action in 2011/12 following the departures of both Wes Brown and John O'Shea to Sunderland.
And Old Trafford supremo Ferguson has acted quickly to ensure the starlet's future in Manchester is secure.
"I am delighted Chris has signed a new contract," he told the club's official website.
"Chris has been outstanding since joining us from Fulham last year, he is an extremely talented young defender, he’s quick, strong and reads the game well, he is a great asset to the team."
Smalling added: "This is a great club and, in the short time I have been here, I have already achieved more than I could have ever expected to.
"I am looking forward to a long and successful career at Manchester United."
