The naked characin (Gymnocharacinus bergii) is a species of freshwater ray-finned fish, a characin belonging to the suborder Characoidei. It is the only species in the genus Gymnocharacinus. This taxon has not been assigned to a particular family within the Characoidei and is considered to be incertae sedis, i.e. its taxonomic affinities are, as yet, unclear. This taxon is the southernmost member of the family (together with some Cheirodon species from Chile) and it is endemic to the headwaters of a stream near Valcheta in northern Patagonia, Argentina, which flow from a thermal spring. It is endangered because of competition from the Uruguay tetra (Cheirodon interruptus) and predation from the rainbow trout (Oncorhynchus mykiss), both being invasive species within the naked tetra's restricted range. The naked tetra attains a maximum total length of . Adults have no scales.