The Spain international is set to move for a fee reported to be in the region of £37 million and will become the club's most high-profile arrival under David Moyes.
Tweets from United's official account on Friday read: "CONFIRMED: #mufc has reached agreement with Chelsea for the transfer of Juan Mata for a club-record fee.
"The deal for Mata is subject to a medical and the agreement of personal terms. A further announcement will be made in due course."
Mata, Chelsea's player of the year for the past two seasons, has been strongly linked with a move away from Stamford Bridge after falling out of favour following the return of Jose Mourinho as manager.
The club struggled to recruit new faces in the previous transfer window, adding only Guillermo Varela and Marouane Fellaini to their ranks.
United's current record signing is Dimitar Berbatov, who arrived from Tottenham in a £30.75m deal in September 2008.
