The former Arsenal captain, 24, kissed the Barça shirt in front of 25,000 fans inside Camp Nou after signing a five-year contract in a €29 million (Dh137.5m) deal.
The Catalans will pay a further €5m if they win two league titles and another Champions League trophy during the course of his contract, while Fabregas will also part with €1m of his own wages each year, which will be paid to Arsenal.
Spanish cities empty during the August holiday season, but a crowd comprised of tourists and locals braved the midday sun on a public holiday to see Fabregas say "hola". He had spent the morning undergoing two medicals before signing the contract.
Fabregas was then ushered to the Sala Paris - a room named after Barca's 2006 Champions League triumph against his former club Arsenal - to speak in more depth about the move, but not before the extended Fabregas family had been marshalled into position on the front two rows by the player's mother.
They watched a video montage of Fabregas starring for Barca's youth teams as he slalomed through rival defences on earth pitches as a 14 year old alongside Gerard Pique and Lionel Messi to Every Breath You Take by The Police.
Barca were angered when Manchester United and Arsenal snared two of their brightest Catalan talents, Fabregas and Pique, aged 16 in 2003. The moves allowed both to flourish before they returned to the club they grew up with and supported, Pique, the central defender, in 2008, and now Fabregas.
The midfielder, who hails from the coastal town of Arenys de Mar, 50 kilometres north of Barcelona, now faces a battle to get into this fine Barca side.
"Competition at Barca?" Fabregas replied to the obvious question posed by one of the hundreds of journalists present.
"Mentally, I feel ready for everything. I chose the most difficult path but that can bring the greatest joy.
"It is always a very hard challenge to come to Barcelona. I have spent many, many hours thinking about the return to Barcelona. This is the greatest team in the history of football."
Asked where he expected to play in Pep Guardiola's starting XI, Fabregas replied: "Position on the pitch? I don't I know, didn't talk with the coach.
"I can add a lot of motivation and hard work.
"I arrive very modestly. I can learn a lot from this coach and these players. I know I will have fun." Fabregas was keen to praise his former club Arsenal, where he rose to great prominence, yet ultimately did not win a single trophy in his final six of eight years at the north London club.
Eulogising Arsene Wenger, his former manager, Fabregas said: "Wenger is a special person, he's a second father for me. He gave me everything and I can never thank him enough.
"I got a bit emotional towards him when I said goodbye and had to send him a message later. Words cannot explain how I feel about him."
Fabregas told his Arsenal teammates on Friday that he was leaving. He expressed his frustration at the lack of trophies, despite his happiness at the Emirates Stadium.
"I spent eight years, a third of my life, at Arsenal," he said.
"It is one of my great regrets that I could not lift a trophy as Arsenal captain.
"It was hard to always come close but not win anything, but Arsenal is a great club with great fans.
"I will watch them as often as I can and I will always be an Arsenal fan."
Fabregas came close to silverware several times following Arsenal's 2005 FA Cup win.
"It's not that we didn't win titles," Fabregas said, "it was the routine of the same thing for many years, the finals we messed up and the semi finals - that was hard to take."
Many Arsenal fans had been unhappy about the manner of his transfer.
"I understand people may be disappointed I left," Fabregas said. "But life moves on. I'm back now and I won't let anyone down, I'll give my last drop of sweat."
Fabregas, who will wear the number 4 shirt with "Fabregas" on the back, rather than "Cesc" as he is known in Catalonia, could face an early baptism of fire.
Barcelona meet Real Madrid in the second leg of the Spanish Super Cup tomorrow night, with the score delicately poised at 2-2 after Sunday's first leg.