The Spaniard took just over two hours to wrap up his third triumph in this clay court tournament, following his other win in 2008.
It was Volandri who started the better of the two, breaking the 26-year-old in the second game of the match.
However, Almagro came roaring back with three breaks to take the first set 6-3.
The Italian then dug deep to save five break points in the second set before taking his chance to level the match, winning through 6-4.
Almagro clinched his 11th ATP Tour title after winning the third set 6-4, becoming the fifth-consecutive Spanish winner in Brazil.
Spain has been represented in the final in each of the last eight years, stretching back from Rafael Nadal's 2005 victory.