He said: "I'm so happy to be here because everyone knows Barcelona are the best team in the world. I'm feeling like a young boy.
"To be part of this team is something I wanted to achieve when I was young.
"I always tried to think 'I will play for Barcelona' and now I have the opportunity to be here and play with the best players in the world - Messi, Xavi, Iniesta, Fabregas."
Song will be available for selection when Barca take on Real Madrid at the Nou Camp in the first leg of the Spanish Super Cup on Thursday evening.