The Blues have moved to tie-down many of their stars on fresh contracts following Fifa's transfer ban.
Salomon Kalou inked a new three-year deal on Monday and now Brazil international Alex has agreed a new four-year contract.
Alex, who was linked with Real Madrid in the summer, has yet to play this season because of injury and he is delighted to pledge his long-term future to Chelsea.
"I am very happy to start again to train and feel no pain, and to sign for four more years," said Alex.
"It was easy to decide to sign and now I hope to get back to playing well."
Blues boss Carlo Ancelotti welcomed the defender's decision to continue his career at Stamford Bridge.
"I am very happy that Alex has signed a new contract with the club," Ancelotti told the club's official website. "He is just returning from injury and surgery but he is a player I know well.
"I am familiar with all of his fantastic qualities, so I am delighted he will be with us for another four years."