Mata will be available to play against Norwich City on Saturday having already trained with the squad, Chelsea said on their website.
Valencia, third in La Liga last season, had wanted around 30 million euros for Mata, according to media reports.
"Right from the first moment there have been good feelings all around me,' Mata told the website. "Training has been great and the players are a great group to train with.
"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."
The 23-year-old left-winger is the third Spanish player to join Chelsea this year. Fellow World Cup winner Fernando Torres joined from Liverpool last season and midfielder Oriol Romeu signed from Barcelona this month.
Mata has 11 caps and been a mainstay of the Valencia side since 2008 with his speed and goals marking him out as one of Europe's most exciting wide men.
Chelsea have also signed Belgium striker Romelu Lukaku in the transfer window as new coach Andre Villas-Boas looks to snatch back the Premier League title from Manchester United.
"He [Villas Boas] said he wanted Chelsea this season to play more attacking football, and that I would be a big part of that," said Mata.
"The plan is that I can play on either flank, or even in the middle, playing in between the lines of midfield and attack. I like to keep the ball and use it well, I am an attacking player and, above all, I like to set up goals and score them also."
