Intraspecific morphological variation is a relatively common pattern among lizards, where several selective factors have been suggested as responsible for this phenomenon. For instance, geographic variation could result from natural selection along with historical processes, whereas sexual dimorphism has usually been attributed to sexual selection, natural selection, and niche segregation. Liolaemus wiegmannii is a diurnal lizard distributed in the center, center-east and north-west of Argentina, as well as on the shores of south-west and south Uruguay. Information about morphological variation in this species is almost entirely limited to differences in mid-body scales between populations in the north and center of Argentina and some sex-based morphometric variation. Herein, we studied the geographic and sexual morphological variation of Liolaemus wiegmannii from Uruguay to test the hypothesis of morphological isolation by distance and morphological structuring by geographic barriers (rivers), as well as exploring the occurrence of sexual dimorphism in morphometry and lepidosis. Neither geographic distance nor rivers seem to play an important structuring role on the external morphology of Liolaemus wiegmannii in Uruguay. Multiple multivariate analyses support the hypothesis that most of the external morphological variation is probably due to sexual dimorphism. Natural and sexual selection acting on females and males, respectively, are the most plausible mechanisms underlying the dimorphism observed in this species.