The 27-year-old Fabregas, who spent eight years at Arsenal before moving to Barcelona, said Thursday that he still has ''unfinished business'' in the English Premier League.
''Now is the right time to return,'' he said. ''You all know that Arsenal was the first team that had the chance to sign me. They decided not to do so, so fate has not led our paths to cross again.''
Fabregas was a key cog of Arsene Wenger's Arsenal side, winning the FA Cup in 2005 and reaching the Champions League final a year later. But now he'll be looking for trophies with bitter rivals Chelsea and manager Jose Mourinho, who has feuded over the years with Wenger.
''I considered all the other offers very carefully and I firmly believe that Chelsea is the best choice,'' Fabregas said on the club's web site. ''They match my footballing ambitions with their hunger and desire to win trophies. They have an incredible squad of players and an extraordinary coach. I am completely committed to the team.''
Media reports in Spain and England put the transfer fee at about 26.5 million pounds ($44.67 million).
During his three years at Barcelona, Fabregas won a Spanish league title, as well as the Copa del Rey, the Spanish Super Cup, the UEFA Super Cup and the Club World Cup.
''I would like to thank everyone at Barcelona where I enjoyed three wonderful years,'' Fabregas said. ''It was my childhood club and I will always be proud and honored that I played for such a great team.
But Barcelona hinted it was unhappy with some of the performances from Fabregas, saying on its website that his standard of play declined as the seasons progressed.
''The good sensations little by little vanished in each of the three seasons. ... His second halves of each season were never like the first halves,'' it said.
Fabregas is currently with the Spanish team in Brazil for the World Cup, where Spain is defending champion. He has also won two European Championships with his country, in 2008 and 2012.
''For me it was fundamental that the signing be finished before the beginning of the World Cup so that I can fully concentrate in helping my country win the World Cup,'' Fabregas said.
He will wear the No. 4 shirt at Chelsea, previously worn by David Luiz, who is set to join French league champion Paris Saint-Germain.
Associated Press Writer Joseph Wilson in Barcelona contributed to this report.
- Sports & Recreation
- Cesc Fabregas