The Marsh Seedeater Sporophila palustris is one of the most endangered and least known of the capuchino seedeaters. Breeding populations are patchy, occurring in north-east Argentina, southern Brazil, and Uruguay. We present data on the breeding biology of the species, and describe nests and eggs, behaviour (including courtship, nesting, incubation and parental care), breeding sites and food items. Marsh Seedeaters breed in well-preserved grasslands with wet soils and tall vegetation. In general, these habitats are used for extensive livestock ranching. Main threats are overgrazing, widespread use of fire, conversion of grasslands to pastures of exotic grasses and rice fields, afforestation, and illegal trapping. Most breeding sites are located in Important Bird Areas, but only one in Brazil and one in Argentina are protected. We propose a series of actions to promote the conservation of Marsh Seedeaters and other endangered birds that coexist in grassland habitats.