An analysis of morphology, vocalization, habitat preference, and distribution revealed that the southern yellowbilled population ascribed to the Plumbeous Seedeater (Sporophila plumbea) is actually an undescribed species. Individuals of this new species are distinguishable from other Sporophila, particularly S. plumbea, by a combination of diagnostic characters: (1) the adult male has a robust, bright yellow bill with an arched culmen, distinguishing it from S. plumbea and other gray seedeaters; (2) the adult male is larger and heavier than S. plumbea; (3) the adult plumage of males is bluish gray (not plumbeous gray, as in S. plumbea); (4) vocalizations include song that is structurally similar to that of S. plumbea but with clear (whistled) introductory syllables and unique call notes. The new seedeater is segregated from other gray seedeaters in its breeding habitat and breeding distribution. It is endemic to Brazil, where it breeds in upland shrub-grasslands associated with Araucaria forests in southern Brazil and migrates northward to the Cerrado to winter.