The snake Oxyrhopus clathratus, which has a restricted distribution within the Atlantic Forest Domain, exhibits great variability in color pattern, shape of the hemipenis, and scalation. The extent of variation has resulted in substantial taxonomic confusion. To date, identification and recognition are problematic, and this is reflected in the synonymy of the species. We examine the type series and specimens from throughout the species' distribution to assess intraspecific morphological variation. Morphological variability is correlated with geographical distribution and elevation. Further, we explore whether the morphological data support recognition of more than one species or not. The results suggest that O. clathratus is a single species that exhibits considerable variation in morphology, including three distinct patterns of coloration (melanistic, melanistic with inconspicuous narrow light bands, and with conspicuous dark and light bands alternating along the body) that overlap geographically. The morphology of the ornamented hemipenes varies between “T-shape” and “Y-shape” forms, which occur without any meaningful or elevational structuring. The loreals are usually absent. Based on our results we designate a lectotype for the species.