Niche partitioning is an important mechanism for allowing ecologically similar species to coexist, contributing to biodiversity and the functioning of ecological communities. Species partition niches by taking advantage of environmental heterogeneity. However, niche partitioning and species coexistence investigations often do not include intraspecific variation or individual differences like sex and body size even though these factors can have important ecological consequences. Such intrapopulation factors can reduce the number of individuals among species that overlap in resource use and potentially facilitate coexistence. Using stable isotopes (δ 13 C and δ 15 N), we quantified dietary differences among three ecologically similar, sympatric watersnake species: Nerodia erythrogaster , N. rhombifer and N. sipedon . Additionally for each species, we determined intraspecific dietary patterns and determined how those within-species patterns may contribute to dietary niche partitioning among species. Nerodia erythrogaster fed more on terrestrial prey, while N. rhombifer fed at higher trophic levels. Females across species fed at higher trophic levels than did males, and isotopic variance differed between the sexes in N. sipedon . Larger watersnakes foraged at higher trophic levels and fed more on terrestrial prey. Each watersnake species had a distinct diet that overlapped to some degree with the other species’ diets, but these diets varied both between sexes and among size groups within species. This inter- and intraspecific dietary variation can facilitate species coexistence by reducing the number of individuals from all species that use the same resources. Intraspecific variation can add important and nuanced layers to the evolution of species coexistence, and research on interspecific niche relationships needs to increasingly consider the effects of these intraspecific variations.