Spain's Almagro bested countryman and No. 8 seed Albert Ramos 6-4, 7-6 (7-4) in one of the day's semifinals, improving to 18-4 lifetime in the Brazil event.
Almagro also took the title in 2008 and carries a 10-4 career finals record into Sunday's match against Volandri.
The Italian, meanwhile, rallied from a set down to record a 5-7, 6-0, 6-2 win over fourth-seeded Brazilian Thomaz Bellucci.
Volandri, a clay-court specialist, hopes to collect his third career crown and first since 2006.