Alecsandro's away goal saw Internacional progress to the Copa Libertadores final despite losing the semi-final second leg 2-1 to Sao Paulo with the tie finishing 2-2 on aggregate.
Alex Silva put Sao Paulo ahead after 30 minutes, but Alecsandro equalised six minutes into the second half. Ricardo Oliveira restored the hosts' lead, but they could not find the third goal they required to go through to the final.
Internacional's victory over their fellow Brazilians means not only will they meet Guadalajara for the trophy, but they will compete in the FIFA Club World Cup in Abu Dhabi later this year, with the Mexican side ineligible to represent South America in the competition. Guadalajara beat Universidad de Chile 3-1 on aggregate in the first semi-final, but the second semi-final was a tighter affair.
Internacional had won the first leg 1-0 and head coach Celso Roth described the second-leg clash as "the game of our lives". Sao Paulo pressed their rivals from the first minute, but needed a mistake from Inter goalkeeper Renan to take the lead. Renan failed to control Hernanes' free-kick and Alex Silva headed in after 30 minutes.
Inter equalised soon after half-time after Andres D'Alessandro's free-kick was deflected into the path of his team-mate Alecsandro. Alecsandro tucked the ball beyond Rogerio Ceni, meaning Sao Paulo required two goals to go through. The first came swiftly as Ricardo Oliveira put the hosts in front for a second time after 53 minutes.
Sao Paulo continued to press forward and the visitors were reduced to ten men when Tinga was sent off with 12 minutes to go. But Sao Paulo could not make their numerical advantage tell as Inter held on.
The 2006 champions will play the first contest of their two-legged final in Mexico, hosting Guadalajara in the second leg in Porto Alegre.