Sex differences in body size and head dimensions are widespread in adult snakes, but because data are scarce for neonates, it is unclear whether differences are present from birth or arise post-natally. Here we analyse patterns of sexual dimorphism in neonates and adults of four species of natricine snakes, Nerodia sipedon, Storeria dekayi, Thamnophis radix, and T. sirtalis. Two measures of body size (snout–vent length, mass), four measures of head morphology (head length, head width, jaw length, and interocular distance), and tail length were obtained from wild-caught adults and from offspring born to wild-caught females. Among neonates, significant sexual dimorphism was found in body size for S. dekayi and T. sirtalis, in head dimensions for S. dekayi, T. radix, and T. sirtalis, and in tail length for all four species. Among adults, significant sexual dimorphism was found in body size, head dimensions, and tail length for all four species. The degree of sexual dimorphism in body size among adults greatly exceeded that among neonates. In contrast, the degree of sexual dimorphism in head dimensions was similar between neonates and adults. The presence of significant sexual dimorphism among neonates suggests that hypotheses regarding the evolutionary significance of sexual dimorphism in snakes should consider newborns as well as adults.