He was given a warm reception when he appeared alongside the other four presidents at a forum event.
Al Jazeera's Gabriel Elizondo in Belem says the presence of the five leaders adds legitimacy to the forum this year.
However, it is not unsurprising that they would attend, as most of those at the forum share the same political world view as the leftist leaders, our correspondent says.
'Leaving the trenches'
Hugo Chavez, the Venezuelan president, said on Thursday after arriving in Brazil that the forum had to work harder on providing solutions, not just debating issues.
"We must leave the trenches while maintaining the flags, fortifying the ideas, and launch a political ideological offensive everywhere," he was quoted by AP as saying.
"This forum can accomplish that."
Rafael Correa, his Ecuador counterpart, said he wanted to see an alternative to capitalism come out of the forum.
"The system has collapsed, this perverse neoliberal system ... and the forum is part of the solution," he told journalists.
The annual forum, now in its ninth year, began in 2001 in the southern Brazilian city of Porto Alegre.