Fabregas, 27, is currently in Brazil with Spain preparing for the World Cup finals but has decided his club future by securing a return to England and London.
The intelligent midfielder joined Arsenal in July 2003 and made over 200 league appearances for the Gunners, captaining Arsene Wenger's side before his return to Barcelona in August 2011 for a fee of £30 million.
With Wenger deciding against taking up an option to sign him back, Fabregas has signed a five-year contract for a fee believed to be in the region of £27m and will wear the No. 4 shirt at Stamford Bridge.
"I do feel that I have unfinished business in the Premier League and now is the right time for me to return," he said on his Facebook page. "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 won one La Liga title and the Copa del Rey in his second stint at Barcelona, having emerged from the club's youth team.
However, Barcelona have chosen to cut the midfielder loose and are signing Sevilla star Ivan Rakitic as a replacement.
"Firstly I would like to thank everyone at FC Barcelona where I enjoyed three wonderful years," Fabregas added. "It was my childhood club and I will always be proud and honoured that I had a chance to play for such a great team.
"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.
"It was extremely important to me that this transfer was completed before the World Cup so that my mind will only be focused on hopefully helping my country try to retain the trophy."
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