The midfielder only signed a new deal last year - but has been rewarded with a lucrative extension after establishing himself as a key member of Ole Gunnar Solskjaer’s rebuild.
The contract also has the option of a further year.
United’s manager said: “Scott has done tremendously well since I joined and has been a vital part of our midfield.
“He brings determination, tenacity and skill to the game and we know that Scott’s attributes will be a major asset to the squad over the coming years.
“As we always say the Academy is the bedrock of the club and having come through the system everyone knows that Scott has shown the exact mentality that you need to succeed at Manchester United.
“I look forward to seeing his continued improvement in the years to come.”
McTominay is considered a potential future captain at United.
He added: “Whilst I understand that we all have so many other things to think about at the moment, I’m so happy to sign this contract and play a part in the future ahead for this team.
“All I have ever known is United and I hope that my passion for the club shows every time I go onto the pitch. I’ll continue to give everything for this club whenever I pull on the shirt.
“I want to thank the manager for the faith he has shown in me and everyone at the club that has helped me to get where I am today. I’m looking forward to finishing this season on a high and hopefully achieving our aims.”
McTominay has been at United since 2012. He made his debut in 2017 against Arsenal and has gone onto make 75 appearances, scoring six goals.