Studies of diet are central to our understanding of organismal biology. We describe the diet of the Red Diamond Rattlesnake (Crotalus ruber) using data collected from museum specimens, live specimens from a field study, road kills, opportunistic behavioral observations, and existing literature. Dietary samples were collected from across the species' range, including Southern California (USA) and Baja Norte and Baja Sur (Mexico). Examination of 272 individuals resulted in 227 prey items recorded from 219 snakes. The diet of C. ruber consisted largely of mammals (91.6%), but also included lizards (7.5%) and birds (0.9%). No ontogenetic shift in prey type was evident, with mammals consumed by all snake size classes. However, adults fed on larger prey than juveniles. Sexual dimorphism existed in snake length, with adult males averaging longer than adult females. Juvenile males consumed larger prey than females, but no sexual differences in prey mass existed for adults when controlling for snake body length. Snakes from coastal populations averaged longer in body length than snakes from desert populations. Coastal snakes consumed a higher proportion of rodents, and prey of larger body mass when controlling for snake length, than snakes from desert populations. The presence of prey was independent of snake collection month, suggesting year-round feeding, as supported by observations of occasional winter feeding by snakes in California. Although C. ruber may scavenge food opportunistically, behavioral observations suggest that it relies heavily on ambushing mammal prey.