The recently described Tropeiro Seedeater, Sporophila beltoni, is a rare long-distance austral migrant songbird that breeds in upland grasslands of southern Brazil. No aspect of its natural history has been studied previously. We studied the natural history of this seedeater in the grasslands of the Araucarian Plateau from 2007 until 2011, focusing on breeding biology and monitoring 133 nests. The breeding cycle lasts for 3.8 months, and the breeding season is correlated with photoperiod and phenology of the grasses; nesting peaks in November and December; the mean clutch size is 2 eggs (1–3); and only the females brood, for 12 days. The nestling period lasts 10 days, and both parents care for the nestlings, although with different roles. The daily estimated survival rate (DSR) of nests, as modeled by the MARK program, was 0.94 and varied temporally in the breeding season. The estimated reproductive success was 20%. The quadratic model best explained the changes in nest survival, coupled with concealment and nest height from the ground. Other factors tested, including year-to-year variation, age of the nest, and species of support plant, did not significantly affect nest survival. Predation was the main cause of nest failure (48%), followed by desertion of nests and trampling by cattle (37%). Multiple breeding attempts (maximum 3) occurred, averaging 1.75 (SE ± 0.17) nests per female in each breeding season. This information on breeding biology and nest survival will aid in management and conservation efforts in grasslands of southern Brazil.