The 27-year-old is currently in Brazil with the national squad but has put pen to paper on a deal that sees him return to England after leaving Arsenal for the Nou Camp in 2011.
He is believed to have signed a five-year contract for a fee in the region of £30million.
With Barcelona having failed to win any silverware last season and the appointment of Luis Enrique as the new head coach, Fabregas was allowed to leave Spain and he revealed that Chelsea was his preferred option from those who had made their interest known.
"I considered all the other offers very carefully and I firmly believe that Chelsea is the best choice," he said.
"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 became a household name in England after developing into a first-team regular at Arsenal and eventually becoming captain.
He joined the Gunners from Barcelona's youth set-up but, despite earning plaudits for his ability, he only ever lifted the FA Cup during his first stay in England.
Now he returns with London rivals Chelsea after confirming Arsenal had not taken up their option to re-sign him.
"Firstly I would like to thank everyone at FC Barcelona where I enjoyed 3 wonderful years," he said.
"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 do feel that I have unfinished business in the Premier League and now is the right time for me to return.
"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."
With fellow Spaniards Xavi and Andreas Iniesta for competition, Fabregas broke into the Barcelona side and ended his first season with four winners' medals before picking up the first league title of his career the following year.
He was the subject of interest from Manchester United last summer when then-manager David Moyes confirmed the Red Devils had approached Barcelona in the hope of concluding a deal.
The signing will be seen as perfect timing for Chelsea following the end of Frank Lampard's long affiliation with the club and he becomes the first addition to Jose Mourinho's squad as the Portuguese looks to improve on last year's trophyless campaign.