Fabregas, who is preparing for the World Cup with the Spain squad in Brazil, is believed to have signed a five-year contract for a fee in the region of £30 million ($50.5 million).
The 27-year-old former star of Chelsea’s London rivals Arsenal said he had “unfinished business” in English football and revealed that Arsenal had decided not to use their option to re-sign him.
He said in a statement: “I considered all the other offers very carefully and I firmly believe that Chelsea is the best choice.
“Yes, everyone knows that Arsenal had the first option to sign me. They decided not to take this option and therefore it wasn’t meant to be. I wish them well in the future.”
Fabregas said it was “extremely important” to him that the move was completed before Spain begin their World Cup defence “so that my mind will only be focused on hopefully helping my country try to retain the trophy”.
With Barcelona having had a rare trophyless season and with Luis Enrique appointed as the new head coach, Fabregas was allowed to leave the Catalan giants and he said Chelsea were an attractive option.
“They match my footballing ambitions with their hunger and desire to win trophies,” he said.
“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 joined Arsenal from Barcelona’s youth set-up in 2003 and eventually became captain, but the only trophy he won in England was the FA Cup.
He left for Barcelona in 2011, but now he returns to the Premier League with Chelsea under Jose Mourinho, in a midfield also featuring Eden Hazard of Belgium and Brazil’s Oscar.
“I do feel that I have unfinished business in the Premier League and now is the right time for me to return,” he said.
He thanked everyone at Barcelona — “my childhood club” — and said: “I will always be proud and honoured that I had a chance to play for such a great team”.