Understanding patterns of resource use is an important aspect of the conservation and management of animal populations. We used stable isotopes of carbon and nitrogen from nail samples of Western Painted Turtles (Chrysemys picta bellii) to examine isotopic niches for individuals in an urban population. Western Painted Turtles are omnivorous, so we predicted that there would be little isotopic niche variation by sex, location, or age class. In contrast to our prediction, isotopic niche size varied up to three times among groups; females inhabiting marsh habitat had the largest niche, whereas females inhabiting creek habitat had the smallest. Isotopic niches overlapped 26–77%, with the least overlap between adult males and females, indicating niche partitioning by sex. Body size and location also contributed to the diversity of resource use. Isotopic mixing models indicated that all turtles consumed low proportions of a variety of prey items, but there were differences among groups. Turtles inhabiting creek habitat consumed higher proportions of chironomid larvae, whereas those in marsh habitat consumed higher proportions of crayfish and amphipods. Our findings indicate that urban turtles take advantage of a wide range of prey, and that aquatic systems with high productivity and diversity are well suited for maintaining turtle populations.