There had been speculation over the Spaniard's future at the club, but he has been reassured by manager Jose Mourinho since returning to training this week.
"I met with Jose on the first day of training," Mata told reporters. "I think he's a great manager who did a really great job last time he was here.
"The break was really good for me. I had time to rest and disconnect from football, but now I am back at Chelsea and really looking forward to the new season.
"I'm really excited about it and I'm very happy to be here."
Chelsea kickoff their Premier League campaign against newly-promoted Hull City on August 18.