"A few weeks ago, I was set to extend my contract with Arsenal and it was taking some time. But when Barcelona arrived, I said yes straight away, because Barcelona is not a team to be turned down and when the train arrives once in a lifetime, you have to take your chance," he told reporters as he was unveiled at a club presentation.
The 25-year-old -- who is related to Cameroon legend Rigobert Song -- had three years left to run on his contract with Arsenal but scrapped that in favour of a five-year deal with the four-time European champions.
"I am happy to be here and it is a pleasure for me to come to Barcelona, because it is a dream come true. Barcelona are the best club in the world, the best team in the world with some of the most talented players in the world," he added.
"It is an immense joy for me and my family," he added. I have two boys who adore (Lionel) Messi the whole year long and only speak about him and it will be a great joy to have the opportunity to play with him.
"Today, I am a Barcelona player and I will do everything to give the best I possibly can, because it is Barcelona and it is a team of winners, a team of champions".
Song becomes the second key player to leave Arsenal after Dutchman Robin van Persie signed with Manchester United last week for 30.4 million euros.
Barcelona opened their La Liga season last Sunday with a 5-1 thumping over Real Sociedad, helped by two goals from Messi.
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