The Sooty Antbird (Hafferia fortis) inhabits Andean foothills and lowland humid forest from Colombia to the western Brazilian Amazon. Despite its wide distribution, many natural history aspects are poorly known. The nest, eggs, and nestlings are described, with no additional nesting information available. Our study provides detailed behavioral information on the nesting biology of H. fortis based on seven nests that were monitored in lowland humid forest in southeastern Peru. Nests were large domes found mostly on slopes or in ravines. The clutch size was always two eggs. Both sexes share daytime incubation and feed the nestlings, but the female exclusively conducted nighttime incubation and brooding. Daytime nest attentiveness was 83 ± 9.5% (SD; n = 13 days), and was similar in both sexes. Nestling period was 13 days, and adults conducted 3.86 feeding trips per day, which increased as nestling developed. Nesting characteristics observed in H. fortis are similar to other Hafferia species, although there is little information about incubation and brooding behavior for the longipes clade, and in general for the Thamnophilidae. Additionally, we described an unusual nest defense behavior during a nocturnal attack by a snake (Oxyrhopus petola).