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.
He has made just 11 Premier League starts this term, but will now seemingly have the chance to relaunch his career with the reigning Premier League champions, who are desperate to improve on their current position of seventh.
United sit 14 points behind leaders Arsenal and are in need of a lift having also been knocked out of both domestic cup competitions this month.
The club struggled to recruit new faces in the previous transfer window, adding only Guillermo Varela and Marouane Fellaini to their ranks.
Mata joined Chelsea from Valencia in June 2011 and has scored 33 goals in 135 appearances for the London club.
United's current record signing is Dimitar Berbatov, who arrived from Tottenham in a £30.75m deal in September 2008.
