The deal ended one of the most protracted transfer sagas of modern times, Fabregas's boyhood club finally getting their man after chasing him for three years.
The Spanish World Cup winner passed his medical and was presented to around 30,000 Barca fans in the Nou Camp as he returned to the club he left as a 16-year-old in 2003.
"I have waited a long time for this moment. It's a very special day," Fabregas told the supporters after signing a five-year contract.
"I know that many of you were disappointed when I left but I am here to take up the greatest challenge of my life. After eight years away I hope to leave you all with many great memories."
Vice-president Josep Maria Bartomeu said Barca paid 29 million euros for the 24-year-old and that five million more would be payable if the side won two La Liga titles and one Champions League with him in the squad.
A further one million euros would be paid to Arsenal each year out of the player's wages.
"It's been much more complicated than we expected," Bartomeu said. "On top of the 29 million euros we were able to augment our original offer thanks to the 'push' Cesc made to the deal."
WENGER PRAISE
Fabregas thanked his new coach Pep Guardiola and his former boss Arsene Wenger who signed him from the Barca youth academy.
"I want to thank Pep Guardiola for the effort he made and the confidence he gave me," Fabregas told reporters. "I am prepared for the move now.
"I wasn't ready two or three years ago. I had various options but I chose the most difficult path. I knew I wanted to be with the best in the world.
"Wenger is a special person. He will always be like a second father because he has given me everything in football," added Fabregas.
"They have a bad image of him here but if I am here it is due in large part to him. I will always have him in my head and my heart."
Fabregas said he had been unable to tell Arsenal fans what was happening, explaining the club had effectively gagged him.
"I'm sorry I could not speak to them and I am sad about leaving as I spent one third of my life there, eight years. It was a sad moment for me but life goes on," he added.
"The biggest regret I will have in my career is I did not lift a trophy as Arsenal captain but I am sure they will be a strong team again."
Fabregas did not play in the London's club Premier League-winning campaign in 2004 but featured in their FA Cup-winning side a year later.
He will have his first training session with Barca later on Monday when the squad prepare for Wednesday's Spanish Super Cup second leg against arch-rivals Real Madrid.
The first leg on Sunday ended in a 2-2 draw.
"I'm prepared to play," Fabregas said. "It all depends on the coach but I am prepared to fight for everything."
