The 50-year-old Martino will follow two of European soccer's most successful coaches Tito Vilanova, who succeeded Pep Guardiola.
Despite having no European coaching experience, Martino received the backing from Messi before the club's sporting director traveled to Buenos Aires to negotiate the deal. Messi spoke glowingly of Martino, better known in Argentina by the nickname "Tata."
"I like Tata Martino," Messi said recently. "He is a great coach," adding that "his teams play well and we all respect him."
Martino acknowledged Messi's influence in landing the job.
"I have no doubt that Jorge (Messi's father) and Lionel have had importance," Martino said at a news conference in Rosario, Argentina. "They surely have spoken with the club officials. Surely they were asked their opinion."
Martino coached Paraguay in 2010 to the quarterfinals of the World Cup for the first time. Barcelona praised him on its website for his "clear commitment to the passing game" and the attacking style that has become the Spanish power's trademark.