The varying influences of selective forces throughout a species' range can result in geographic variation in sexual size dimorphism (SSD). The Moroccan turtle, Mauremys leprosa, occupies an extremely wide variety of ecoregions and habitats, including coastal rivers, mountain streams, oases, and intermittent rivers in the northern fringe of the Sahara Desert. To assess geographic variation in SSD, we collected specimens along an environmental gradient in central Morocco, including Oued (River) Ksob on the Atlantic coast, Oued Zat in the High Atlas Mountain foothills, and Oued Drâa in the Sahara Desert. Only turtles with conspicuous secondary sexual characteristics were included in our analysis. We calculated a sexual dimorphism index (SDI) using the mean size of the larger sex divided by the mean size of the smaller sex and subtracted one from that ratio. The direction of SSD was biphasic: in one population males and females exhibited the same body size, whereas, in two other populations, females were larger than males. Mean straight-line carapace lengths of males and females were not statistically different at Oued Ksob (SDI  =  0.08), and females were relatively small. In contrast, females from Oued Zat were significantly larger than males (SDI  =  0.56) and females were larger than those from Oued Ksob. SSD was most dramatic at Oued Drâa (SDI  =  0.92) and much greater than any value previously reported for the species, with females exhibiting a mean carapace length greater than those in the other 2 rivers. A 2 × 3 factorial analysis of variance that compared the mean size of the sexes among the 3 sites yielded a significant SEX × LOCALITY interaction (p < 0.001), which demonstrated geographic variation in SSD. Geographic variation in SSD appears to represent the interaction of natural and sexual selection on growth rates and maturity schedules of males and females in different environments.