Here we report on the discovery of a new population of red uakaris in the mountains of northern San Martin, north-eastern Peru. This population is isolated from the other known uakari populations in the eastern lowlands, which raises questions concerning their taxonomic status and biogeographical history. This follows a recent range extension of this taxon west of the Ucayali River. Previously, the Peruvian red uakari (Cacajao calvus ucayalii) was only known in Peru from the lowlands between the Amazon, Ucayali and Yavarí Rivers.