Morphological and molecular evidence reveals a new species of Characidium from the Ucayali-Urubamba Piedmont, Peru, and novel molecular clades are proposed within the genus.

A new species of Characidium is described from the Urubamba River basin, within the Ucayali-Urubamba Piedmont ecoregion, Peru. The new species can be readily distinguished from all congeners, except Characidium cacah, Characidium chicoi, Characidium helmeri, Characidium mirim, Characidium nana, Characidium nupelia, Characidium sterbai, Characidium stigmosum and Characidium xavante by the possession of an incomplete lateral line and the lack of an adipose fin. It differs from these species, and remaining congeners, by a unique sexually dimorphic colour pattern: females exhibit irregular dark blotches along the dorsum that alternate elongation between the sides of the body and are usually disconnected from the lateral bars, whereas in males the dark bars are absent. Additional diagnostic features include two series of dentary teeth, absence of a dark blotch on the caudal peduncle, 12 circumpeduncular scales and a fully scaled isthmus. Molecular data indicate a significant genetic divergence between the new species and its closest relatives, further confirming its recognition as a distinct species. Additionally, two new clades were recovered in molecular phylogeny, each formed mostly by species not previously included in any molecular phylogenetic studies.