The former Arsenal midfielder is returning to the English Premier League after three seasons at Camp Nou.
Jose Mourinho's side is believed to have paid 30 million pounds for the Spain international, who has signed a five-year contract at Stamford Bridge.
Fabregas scored eight La Liga goals for Barcelona and provided 19 assists in 36 appearances last season and thanked the Catalan giants after sealing his move to Stamford Bridge.
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Fabregas told the club's official website: "Firstly I would like to thank everyone at FC Barcelona where I enjoyed three wonderful years. It was my childhood club and I will always be proud and honored that I had a chance to play for such a great team.
"I do feel that I have unfinished business in the Premier League and now is the right time to return.
"I considered all the other offers very carefully and I firmly believe that Chelsea is the best choice. They match my footballing ambitions with their hunger and desire to win trophies. They have an amazing squad of players and an incredible manager. I am fully committed to this team and I can't wait to start playing."
Fabregas spent eight years at Arsenal before joining Barcelona.