Juan Mata completed his medical at Manchester United’s AON training facility on Saturday before penning down for the Premier League champions for a £37.1m club-record fee paid to Chelsea.
The Spanish midfielder, who scored 32 goals in 135 appearances for the Blues, said he is “thrilled” to join a big club like United.
He has signed an agreement understood to run until summer 2018.
“I am thrilled to be joining United. I have enjoyed some very happy years at Chelsea but the time has come for a new challenge. United is the perfect place for me and I am excited at the chance I have to be part of the next phase in the club’s history.
“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.”
Most of United players took to twitter to welcome the 25-year-old while his fellow countryman David de Gea was at the venue to watch him arrive in a helicopter.
“He (De Gea) has been texting me in the last few days saying, ‘when are you coming, when are you landing?!’ ‘Are you coming by helicopter or by bicycle?!’ I am here now and I am very happy to be here, particularly with David,” Mata later told MUTV in his first exclusive interview.
Manager David Moyes could quickly thrust the attacking midfielder into action on Tuesday when another former Red Devil Ole Gunnar Solskjaer returns to the club as Cardiff City boss in a league tie.
Moyes said: “I have been impressed with the excitement he has shown to be part of the new Manchester United. I know our fans have admired Juan for some time now – he has always played well against us – and I know they will be delighted to see him in the red of United, starting on Tuesday.”
Mata, who will be unveiled at a news conference on Monday at 13:30 GMT, becomes Moyes’ second high-profile signing following the £27m arrival of Marouane Fellaini from Everton in September.
He was voted Chelsea’s official player of the year in the past two seasons since joining from Valencia for £23.5m in August 2011, but has found it difficult to earn a starting role for himself under the returning Jose Mourinho.
“He was fantastic for Chelsea and Chelsea was fantastic for him. We are not afraid he is going there to do very well. We want him to go there and be happy,” said Mourinho.
“I asked him when the offer came whether he really was happy to go and he said ‘yes, I am really happy to go’. After that we could feel mixed emotions.
“This is a fantastic opportunity for him. Another big club wants him in the country he want to stay in, has made the right offer for him, so, although lots of clubs feel you should not sell to rivals, we are different.”