Lionel Messi, speaking last year
Argentina star Messi is a big fan of Martino and was rooting for his countryman to replace Tito Vilanova, who has stepped down to battle cancer.
Messi and Martino both hail from Rosario and his appointment is intended to keep the four-time Ballon d'Or winner happy.
Speaking a year ago, Messi said: "I like Martino. He is a great coach. He gets his teams playing well and we all respect him."
Martino, 50, led Newell's Old Boys to the Argentinian title last season but recently severed ties with the club. He has agreed a two-year contract at Barca.