Limited knowledge about species diversity restricts inferences about biogeography and macroecology in Amazonia, where biodiversity documentation has become particularly pressing due to anthropogenic impacts. An emblematic example of poorly known Amazonian amphibian diversity is that of Allobates nurse frogs, in which species boundaries and ranges are unclear. Based on genetic, morphological and call data from samples collected in 14 poorly known sites in Brazil, we identify and describe a new, broadly distributed western Amazonian nurse frog. In the light of the genetic data, combined with examination of more than 400 museum specimens, we also assess the distribution and levels of phenotypic variation in poorly known species from Amazonian lowlands, namely A. flaviventris, A. gasconi, A. subfolionidificans, and A. trilineatus. Lastly, our morphological examinations point to inconsistencies and heterospecific samples in the type series of A. fuscellus, A. gasconi, and A. vanzolinius, as revealed by large variation in key traits. By improving knowledge about species ranges and identifications, we hope that this study will support biodiversity inventories and stimulate further investigations of understudied western Amazonian nurse frogs.