The 32-year-old has been expected to make the move to Stamford Bridge after arriving in London this week to complete a medical.
The Cameroon international, who played under Blues boss Jose Mourinho during his stint at Inter Milan, has put pen to paper on a one-year contract.
"I am very happy to be here but also anxious because I want to get playing as soon as possible," Eto'o told the club's official website.
"It wasn't a hard decision. I saw the qualities Chelsea have, and I was very happy with Jose Mourinho before, so when the opportunity came, I was very happy to take it."
Eto'o's arrival pours doubt over the potential swoop of Manchester United striker Wayne Rooney, who was the subject of two rejected bids from the Blues during the summer.