The 18-year-old has penned a deal keeping him at Old Trafford until June 2023, with the option to extend for a further year.
Greenwood became United’s youngest ever starter in the Premier League era against Cardiff last season and has played an increased role under Ole Gunnar Solskjaer this campaign.
“Having grown up as a Manchester United fan, playing for the first team is a real dream come true.
“I know that, when I play for this club, I represent the whole Academy and I want to repay the faith that my coaches have shown in me with my performances on the pitch.
“I am learning all the time from the manager and his coaching staff and I know that this is the perfect club to help me reach my potential,” Greenwood told the club’s official website.
