The North American genus Thamnophis is a widely distributed and abundant group of snakes. Previous research has indicated that most garter snakes are generalist predators that largely consume anurans, with some degree of geographic and temporal plasticity in response to resource variation. We studied the dietary habits of the Plains Garter Snake, Thamnophis radix, based on stomach contents from 548 animals collected in the field from 1995–2006 at the northern limit of the species' range in central Alberta, Canada. Feeding frequency overall was low (22%) and was highest in spring and early summer. Diet was composed mainly of anurans (85%), especially Wood Frogs (Rana sylvatica) and Boreal Chorus Frogs (Pseudacris maculata), and occasionally invertebrates (10%) and small mammals (5%). Small snakes (<200 mm) began feeding shortly after birth and consumed mainly smaller frogs. Larger snakes ate a wider range of prey sizes and types, but frogs still constituted the majority of prey consumed. Despite sexual size dimorphism, males and females ate prey of the same size and type; however, gravid females contained food less frequently than both males and nongravid females. This study adds to the growing number of dietary studies on T. radix and provides additional evidence for the ecological plasticity of Thamnophis species. Further studies are needed to examine how important these snakes are as components of wetland food chains.