Nemanja Matic thanked Chelsea fans after completing a £40m move to Manchester United.
It ended Matic’s second spell at Stamford Bridge and sees him reunited with United’s former Blues boss Jose Mourinho at Old Trafford.
The Serbian midfielder, 28, has signed a three-year contract.
“I am delighted to have joined Manchester United. To work with Jose Mourinho once again was an opportunity I couldn’t turn down,” he told United’s website.
“I have enjoyed my time at Chelsea and would like to thank the club and the fans for their support. I cannot wait to meet my new team-mates and to start training with them.
“This is a very exciting time for the club and I am looking forward to playing my part in creating more history for this great club.”
Matic returned to Chelsea in January 2014 when Mourinho spent £21m to sign him from Benfica.
He joined the Portuguese club three years earlier as part of the deal which saw David Luiz move to SW6.
Matic played a key role in Chelsea’s Premier League title triumph under Mourinho in 2015 and helped them regain the trophy under Antonio Conte last season.
Mourinho said: “Nemanja is a Manchester United player and a Jose Mourinho player. He represents everything we want in a footballer; loyalty, consistency, ambition, team player.
“I would like to thank him for his desire to join us because without that, it would be impossible to have him here. I am sure our players and supporters will love him.”