Maranhão Hermit Phaethornis maranhaoensis is endemic to Brazil, where it occurs in the states of Piauí, Maranhão, Tocantins, Pará, Mato Grosso and Goiás. Nothing has been published concerning its breeding biology. We present the first descriptions of the nest, eggs and nestlings of P. maranhaoensis, with data on nestling development. We found four nests in the understorey of closed-canopy forest in eastern Maranhão. All four nests were attached to the undersides of babaçu palm fronds (Attalea speciosa), and were constructed of plant material and moss, bound together with spider webs. Nests are similar to those of other Phaethornis, conical and attached to the tip of the frond. They were sited at a mean height of 71.5 ± 21.3 cm above ground, and were 23.6 ± 1.8 cm in height, with an external diameter of 41.7 ± 2.7 mm, internal diameter 18.4 ± 3.7 mm, and the incubation chamber was 24.5 ± 3.1 mm deep (n = 4). Eggs are white and elliptical, measuring 11.9 ± 0.2 × 7.8 ± 0.1 mm, with a mean mass of 0.4 ± 0.05 g (n = 4). Our observations indicate that the species' breeding season occupies November–April.