"Manchester United has been my life since the age of five and playing for the club I've always supported is all I've ever wanted to do," McTominay told United's official website.
McTominay, who has won five Scotland caps, was linked with a loan move away from Old Trafford in January until a calf injury to fellow midfielder Marouane Fellaini put a halt to those plans.
"Scott is a young midfielder and is constantly improving. He's an intelligent player and wants to learn all the time," Manchester United caretaker manager Ole Gunnar Solskjaer said.
"I am delighted he has signed a new contract and there is no doubt that he has a bright future ahead of him."
The club have the option to extend McTominay's deal for a further year once his contract ends.
(Reporting by Simon Jennings in Bengaluru; Editing by Ian Chadband)
