We present the first detailed description of the nest and eggs of the Black-bellied Seedeater (Sporophila melanogaster) based on observation of 64 nests in three areas of grassland in southeastern Santa Catarina and northeastern Rio Grande do Sul states, Brazil. The nests were found from November through March. The nest has the shape of a shallow basket, constructed with portions of dried grasses and strung with spider webs. Half of the nests were constructed in Ludwigia sericea (Onagraceae), and the other half were in 15 other plant species. Of these, the most important were Eupatorium polystachyum (10%) and Achyrocline vauthieriana (6%) (Asteraceae). Only the female constructs the nest. The eggs are ovoid and colored whitish with brownish-purple spots near the wider portion. Clutch size was two eggs, rarely three.