Smalling, 25, joined United from Fulham in a £10 million ($14.9 million, 13.9 million euros) deal five years ago and is enjoying his best season to date at Old Trafford, having made 23 appearances and scoring four goals.
“Chris has improved and developed immensely during the short time that I have been at the club and has become an integral part of the first-team squad,” United manager Louis van Gaal told the club’s website.
“He always conducts himself in a very professional manner and it is also pleasing that he has scored some important goals this season, too. I am delighted he has signed a new contract.”
Smalling, who can also play at right-back, has made 147 appearances for United, scoring nine goals, and has won two Premier League titles with the club.
His opportunities at Old Trafford were initially limited by the presence of Rio Ferdinand and Nemanja Vidic, but with both players having left the club last year, he has become an important member of United’s defence.
“I’m delighted to sign my new contract. I am so proud to play for such a great club,” said Smalling, whose previous contract had been due to expire next year.
“We have developed as a team over the course of the season under Louis van Gaal and we are now playing as a complete unit. This is a very exciting time to be at Manchester United.
“We are all determined to get back to where we belong and competing at the highest level.”