Mata, 23, hit eight goals last season to help Valencia into third spot in La Liga, having been part of Spain's 2010 World Cup-winning squad.
He also skippered the Spanish Under-21s to European Championship glory this summer.
He has vowed to learn English quickly and expects fellow-countryman Fernando Torres to get him up to speed.
Mata is also delighted Chelsea play in blue, like his first club Real Oviedo.
He told The Sun: "From being a little boy my colour was always blue.
"I loved playing for Real Oviedo and would like to return there one day."