Cesc Fabregas' transfer to Barcelona is complete after the Spain midfielder signed a five-year contract Monday.
Barcelona said it paid $42 million to Arsenal, and incentives could increase the figure to $56 million.
Fabregas himself will pay the Gunners $1.4 million out of his salary for each season of the deal, and the Spanish champions will pay up to $7 million more depending on trophies won.
The Spanish club said Fabregas passed a physical and then an "exhaustive" fitness test with club doctors before signing his new deal that brings the 24-year-old back to his boyhood club after eight seasons with Arsenal.
"It took a lot of years, months, days to get here ... it's nice to be home after eight years," Fabregas said in Catalan during his presentation to fans at the Camp Nou stadium, where he continuously kissed the Barcelona crest. "This is the most difficult challenge of my life."
Fabregas is expected to train with his new teammates later Monday, when he would get his first taste of where exactly he will fit into Barcelona's talented squad, which already includes Lionel Messi, Xavi Hernandez, Andres Iniesta and Alexis Sanchez.
Fabregas, who returns to the club that nurtured him before he joined Arsenal in 2003, came onto the field wearing the No. 4 jersey before dribbling the ball and saluting fans, continuously patting the Barcelona crest between kisses and looking overwhelmed by the long-awaited return.
Many fans carried signs reading "Cesc welcome home" as they watched him don the navy and crimson jersey as a Barcelona player for the first time since 2003.
Fabregas announced his desire to move back to Barcelona after the 2009-10 season, but Arsenal refused to sell its most valuable player, and Fabregas agreed to stay for one more year. A similar story line played out this offseason, with the Arsenal captain again stating his desire to return to Spain, and Arsenal trying to fend off Barcelona's interest.
"There have been some long weeks, months, of suffering. Not the worst suffering of my life, but yes these have been difficult days not knowing what my future would be," said Fabregas, who will wear the same number as his boyhood idol and current coach Pep Guardiola. "It appeared it would never end."
Fabregas sat out most of Arsenal's preseason, with the club citing injury concerns, and he was not selected for the team's Premier League opener on Saturday — a 0-0 draw at Newcastle.
Fabregas scored 57 goals in 303 appearances and won one league title and an FA Cup with Arsenal. He has played 58 times for the Spanish national team and was part of the squads that won the 2010 World Cup and 2008 European championship.
Fabregas could make his debut Wednesday at the Camp Nou as the Spanish champions play Real Madrid in the return leg of the Spanish Supercup after a 2-2 draw in Sunday's first leg.
AP Sports Writer Paul Logothetis in Madrid contributed to this report.