Geographic variation in body size may reflect adaptations to local environments, and sexual size dimorphism (SSD) arises from ultimate and proximate factors acting differently on males and females in those environments. The Painted Turtle (Chrysemys picta) is a wide-ranging North American freshwater turtle species with known female-biased SSD. We hypothesized that, in more seasonal environments, the disparity between adult female and male body size would be more pronounced (i.e., the sexual dimorphism index [SDI, female body size/male body size] would be higher) than in more moderate environments because selective pressures on females to maximize reproductive output would result in relatively larger body sizes (fecundity advantage hypothesis) in extreme environments. We predicted that the SDI would be higher in populations at northern latitudes and middle longitudes than in southern and coastal populations. We conducted linear and nonlinear regression analyses using data from the literature and museum records, extrapolated data, and unpublished data on adult male and female carapace and plastron lengths from 65 locations. In contrast to our prediction, SDI decreased with increasing latitude. With respect to longitude, the trend supported our prediction in that the SDI was slightly higher for interior populations and lower for coastal populations; however, the relationship was not significant. Future research should examine sex differences in carapace height and body volume which may more directly reflect selective pressures on female fecundity than straight-line shell lengths.