The 30-year-old, who has been plagued by injuries in recent years, spent the last campaign on loan at Real Madrid.
"I am ready to give my all to Chelsea," Sky Sports News quotes Essien as saying. "I haven't lost any desire or motivation.
"I have been working hard in the last few weeks to be physically and mentally fit for the season and I feel good about my work so far."
Chelsea start the new Premier League season at home to Hull City on August 18.