We describe a new species of capuchino of the genus Sporophila (Emberizidae) of the Esteros del Ibera, province of Corrientes, in northeastern Argentina. This species would have remained unidentified due to lack of ornithological explorations in the central area of the Esteros del Ibera. It has been confused with immature individuals of other Sporophila species. We made observations of behavior and habitat, playback experiments, comparative analyzes of the vocalizations and plumage with other sympatric species of the same genus, and we found this species, which we have named Sporophila iberaensis, inhabiting wet grasslands from the edges of the marshes, having a unique vocal repertoire and a unique plumage. Because of its restricted geographical distribution and the threats that have their habitat, this new species should be categorized as Vulnerable on the IUCN Red List.