The Brazil international is the latest of several Chelsea starts to sign new deals, following Fifa’s transfer ban.
Alex has been out of action so far this season due to an injury.
“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.”
“I am very happy that Alex has signed a new contract with the club,” Blues boss Carlo 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.”