Alex is the latest Chelsea player to sign a new long-term contract with Carlo Ancelotti's side.
The 27-year-old Brazilian has penned a four-year deal with last season's FA Cup winners.
Alex, who has only just recovered from groin surgery, told the Blues' official website: "I am very happy to start again to train and feel no pain, and to sign for four more years.
"It was easy to decide to sign and now I hope to get back to playing well. My condition now is so-so. I think in two more weeks I am 100 per cent and can play without a problem.
"I hope now to start to play in the Chelsea games and play very well.
"To get back into the national team is important to me. Also the Chelsea team started very well this season, is at the top, and this season to win the Premier League is important to me. Now I want to play."
Alex has scored seven goals in 80 appearances for the club following his transfer from PSV Eindhoven in the summer of 2007 but has not featured in the first team so far this season.
Ivory Coast international Salomon Kalou signed a new three-year contract with the club on Monday.