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.