The 21-year-old Alejandro Garnacho's sale value is the fourth largest for a United player after Cristiano Ronaldo, Romelu Lukaku and Angel di Maria.
Chelsea have signed Argentine winger Alejandro Garnacho from Manchester United on a seven-year contract, the Premier League club said on Saturday.
A source close to United told Reuters on Thursday that United had agreed a deal worth 40 million pounds ($54 million) with the Club World Cup champions.
The 21-year-old Garnacho's sale value is the fourth largest for a United player after Cristiano Ronaldo, Romelu Lukaku and Angel di Maria.
"It's an incredible moment for my family and I to join this great club. I can't wait to get started," Garnacho said in a statement.
"I watched the Club World Cup and to join the world champions is special – we're the best team in the world! It’s amazing to be here and I'm very happy."
Garnacho, who scored 11 goals and provided 10 assists for United last season, made 144 appearances in all competitions for them, scoring 26 goals.
The Argentina international was among five players seeking to exit the club this summer, along with Marcus Rashford, Antony, Tyrell Malacia and Jadon Sancho. Rashford joined Barcelona on loan last month.
Chelsea, who beat Fulham 2-0 in the Premier League on Friday, visit Brentford on September 13.