The teenager is the son of former Brazil international Mazinho, who now works with Barca's junior teams, and signed a three-year deal as soon as he turned 16.
"As soon as I turned 16 things moved very quickly," said Alcantara. "Chelsea made an offer, but I preferred to sign a professional contract with Barca. I am happy here. The teams play so that everyone touches the ball a lot. I like that sort of style."