The southern capuchino seedeaters constitute a rapid radiation of finch-like birds that inhabit Neotropical grasslands in continental South America. While capuchinos are highly sympatric, the mechanisms allowing them to breed in sympatry, such as patterns of microhabitat use, remain largely unknown. We provide the first nesting account of the Iberá Seedeater (Sporophila iberaensis), a recently discovered capuchino species that breeds exclusively in the Iberá wetlands of northeastern Argentina, and compare its nesting habits to those of other members of the capuchino radiation. From November 2016 to January 2017, we located and monitored 25 Iberá Seedeater nests. The Iberá Seedeater constructs open-cup nests in clumps of grass on the margins of flooded habitat and lays an average of 2 eggs per clutch. Like other capuchino species, females participate in nest construction and incubation, while both sexes provision offspring. Despite similarities in nest architecture, the Iberá Seedeater differs in nesting habitat from the Tawny-bellied Seedeater (S. hypoxantha), its most abundant congener, and other members of the radiation. Many capuchino species, including the Iberá Seedeater, have been classified as endangered or threatened because of habitat loss and trapping for the pet trade. Further information on the breeding biology and habitat requirements of capuchinos will provide insight into the mechanisms that maintain their coexistence in sympatry and inform conservation efforts to protect this enigmatic group.