Juan Mata has completed his £37.1m move from Chelsea to Manchester United in a record transfer deal for the reigning Premier League champions.
The Spain midfielder had a medical on Saturday and has signed a contract, understood to run until summer 2018.
Mata, 25, is likely to make his debut at home against Cardiff on Tuesday.
"The time has come for a new challenge," he said. "I am excited at the chance I have to be part of the next phase in the club's history."
He added: "Chelsea is a top club and I have many friends there but you cannot turn down the chance to join Manchester United. I look forward to helping the manager and the team be successful in the coming years."
Mata, capped 32 times by Spain, will be unveiled at a news conference on Monday at 13:30 GMT.
He becomes manager David Moyes's second high-profile signing, following the arrival in September of Everton's Marouane Fellaini for £27.5m.
The deal also eclipses United's previous record signing - Dimitar Berbatov from Tottenham in 2008 - by £6.35m.
Mata, who joined Chelsea from Valencia for £23.5m in August 2011, has been Chelsea's player of the year in both full seasons he was at Stamford Bridge.
He scored 32 goals for the Blues and made more than 130 appearances but has failed to command a regular starting place under Jose Mourinho, who returned to Chelsea last summer for a second spell as manager.
"Juan is one of the finest playmakers in the game today and it's a real pleasure to have secured his signature," said Moyes.
