Perithemis mooma, Kirby, 1889 and Perithemis icteroptera (Selys in Sagra, 1857) live in sympatry from southern Brazil to central Argentina. The taxonomy of the genus Perithemis Hagen, 1861 has been hampered by the use of characters that are highly variable or show slight differences among species. Our objective was to assess the efficiency of traditional morphometrics (TM) and geometric morphometrics (GM) to discriminate between these species using wing size and shape and vulvar lamina contour, and to analyze the presence of sexual dimorphism in wing size and shape in both species. The TM and landmark-GM methods were applied on the fore and hind wings, while the outline-based GM method was applied on the vulvar lamina. GM allowed species delimitation using shape variables of either wing. The wing and vulvar lamina shapes were confirmed to be good diagnostic characters to separate these species and appear to be promising tools for distinguishing among other species of this genus. Centroid size failed to achieve species separation. Both species exhibited sexual size dimorphism (SSD). In contrast to what would be expected for Perithemis whose males are strongly territorial, P. icteroptera and P. mooma showed female-biased SSD suggesting a common pattern in Perithemis.