The Cameroon international successfully passed his medical at Barca on Monday and put pen to paper on a five-year contract with the La Liga giants.
Song, whose contract has a buy-out clause of 80million euros (£63million), will now join his new team-mates for the official team presentation at the Joan Gamper Trophy at the Nou Camp on Monday evening, where the Catalan club face Sampdoria.
After passing his medical with flying colours, doctor Ricard Pruna said of Song: "His body fat percentage is extremely low and he has good muscle quality.
"I don't think he's had muscle injuries since he was 17, and that's really good news. He passed all the tests and he's available to the manager."
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The 24-year-old becomes Barca's second major signing of the summer after the capture of Spain full-back Jordi Alba from Valencia and follows a well-worn path from Arsenal to the Nou Camp.
During Arsene Wenger's reign at the Gunners, Marc Overmars, Emmanuel Petit, Giovanni van Bronckhorst, Thierry Henry, Alexander Hleb and Cesc Fabregas have all made the switch to Barca.
It remains to be seen when Song will make his debut for his new side.
After Monday's match against Sampdoria, Barca are back in action on Thursday when they host arch rivals Real Madrid in the first leg of the Spanish Supercopa.
On Sunday Barca travel to Osasuna for their second La Liga game, having started their campaign with a 5-1 mauling of Real Sociedad on Saturday night in what was new coach Tito Vilanova's competitive debut having replaced Pep Guardiola this summer.
Song sat out Arsenal's Barclays Premier League opener against Sunderland on Saturday amid widespread speculation he was on the verge of a move to Barca, and immediately after that match it was announced that the two clubs had agreed terms for the transfer of the player.
Primarily a defensive midfielder, Song joined the Gunners on loan from French club Bastia in August 2005 before the move was made permanent at the end of the 2005/06 season.
In the following season he spent the second half of the campaign on loan at Charlton before returning to Arsenal and subsequently establishing himself as a key member of Wenger's squad.
In total, Song made 204 competitive appearances for Arsenal during his seven seasons at the club, scoring 10 goals.
Arsenal said in a statement on their website: "Everyone at Arsenal would like to thank Alex for his contribution to the club and wish him the best of luck for the future."