The 25-year-old England centre-back had been linked with a move to Arsenal after struggling to hold down a first-team place at United and being branded ‘stupid’ by manager Louis van Gaal for his sending-off in the Manchester derby at the Etihad earlier this season.
Smalling was also guilty of several off-the-field indiscretions, including being pictured going to a fancy-dress party as a suicide bomber last year.
But the defender has matured under Van Gaal and become a regular starter in the Dutchman’s team this season. He was due to be out of contract next summer but has now committed himself to United until 2019.
Van Gaal said: ‘I am delighted he has signed a new contract. 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.