London, England – Spain international Juan Mata completed his year contract with the Blues.
On Monday, the two clubs agreed to a transfer fee of over $40 million, while the 23-year-old wrapped up the move by passing a medical and agreeing to personal terms on Wednesday.
"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," Mata told Chelsea'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."
Mata scored eight goals in 33 league appearances for Valencia last season, and he joins Oriol Romeu, Romelu Lukaku and Thibaut Courtois as new arrivals at Stamford Bridge this summer.