We provide the first detailed nesting biology information for the Yellow-breasted Warbling-Antbird (Hypocnemis subflava), a member of the recently split Warbling Antbird complex. We found seven pouch-shaped nests with two eggs or nestlings within Manu National Park, SE Peru. Long incubation sessions (93.7 ± 7.78 minutes, range  =  4–480 minutes, n  =  18 days) were untaken by both parents and high diurnal nest attentiveness (% of time on the nest) averaged 85.8 ± 1.4% (n  =  16) as a result of taking fewer short foraging trips (6.3 ± 0.6 trips/day, range  =  3–11; n  =  14) that lasted 16.19 ± 1.78 min (range  =  1–91; n  =  18). Incubation behavior produced an average inner nest temperature of 32.3 ± 0.24 °C (n  =  15 days) and average egg temperature of 36.4 ± 0.09 °C (n  =  3). Individual nests at different elevations exhibit differences in incubation behavior. The nestling period was 11 days (n  =  1) and both parents brooded and fed the nestlings. Nest shape, location, and composition were similar to other species in the complex, but egg coloration was variable among species.