"The transfer of Alex Song to Barcelona has been completed for an undisclosed fee. Everyone at Arsenal would like to thank Alex for his contribution to the club and wish him the best of luck for the future," the Gunners said on their official website.
The 25-year-old, who has agreed a five-year contract, became Barca's second major reinforcement for the new season after Spain full-back Jordi Alba was landed from Valencia for 14 million euros.
Song will be presented to fans at the Nou Camp later in the day when Barca take on Sampdoria in the annual Joan Gamper trophy, a traditional friendly.
Barca opened their 2012/13 La Liga campaign with a 5-1 rout of Real Sociedad at home on Sunday.
After the match, new Barca coach Tito Vilanova welcomed the arrival of his new recruit.
"He is going to help us a lot and he brings height to the team," Vilanova told a news conference.
"He will be a surprise to many in football terms because he is a very good player."
Song brings cover to the midfield holding role usually occupied by Sergio Busquets, with Seydou Keita having departed last month, and may be expected to help out in the centre of defence as well.
Barca have been short at the back recently with ageing captain Carles Puyol, who is 34, struggling with a string of injuries and Eric Abidal out as he recovers from a liver transplant.
Busquets and Javier Mascherano have dropped back from midfield to play alongside Gerard Pique in defence at times.
Song joined Arsenal in 2005 and made 204 appearances for the London club.
He follows Dutch striker Robin van Persie in quitting Arsenal this month, leaving fans of Arsene Wenger's team aghast at the continual drain of talent from Emirates Stadium. Cesc Fabregas moved back to Barca from Arsenal last year.
Van Persie joined Manchester United last week in a deal worth 24 million pounds.
