Plain-breasted Piculet is resident is a restricted area in the western Amazon Basin. The distribution of this species is centered on the Río Ucayali in eastern Peru, and from there downstream along the Amazon (Short 1982, Schulenberg et al. 2010). It occurs in extreme southeastern Colombia along the Amazon (Remsen 1977, Hilty and Brown 1986). There is a single record for western Brazil at Maguiri, on the right bank of the rio Japurá, Amazonas (Sick 1997); its presence here suggests that Plain-breasted Piculet should occur along the Amazon east at least to Tefé.
Plain-breasted Piculet is entirely confined to the lowlands.