The 23-year old winger has passed a medical and agreed personal terms with the Blues after Valencia and the Londoners came to an agreement on a fee of £26million on Monday.
Mata, who began his career with Real Madrid, is delighted that the deal has now been done and is excited about the next chapter of his career.
“Right from the first moment there have been good feelings all around me. Training has been great and the players are a great group to train with,” he told the club’s official website.
"Valencia was a big club anyway but I have left to join a bigger club and it is all about the opportunity to win trophies and I have that here.
"I want to adapt as quickly as possible to my new surroundings, my colleagues and this league, and I would love to win a trophy in my first season. If it can be the Premier League then great."