Pristimantis is the amphibian genus with the largest number of described species yet harbouring many species still to be named, notably in Amazonia. Here, I describe a member of the P. diadematus species complex from southwestern Amazonia in Brazil based on external morphology. The species is most similar to P. divnae, a species from southern Peru, but diagnosable from it by the combination of the following characters: presence of nuptial pads and tympanum, smaller body size (SVL 19.5-23.5 mm in males, 27.8-30.6 mm in females), and absence of yellow marks in both the inguinal region and shanks. The new species occupies the same habitats as the syntopic P. altamazonicus, another member of the P. diadematus species complex. Based on an extensive literature review, a new composition of the P. diadematus species complex is proposed and discussed.