Dietary studies are central to our understanding of organismal biology. We describe the diet and frequency of feeding in the Southwestern Speckled Rattlesnake (Crotalus pyrrhus). We used data from wild-caught specimens collected from six biogeographic regions (pooled to three regions for analysis) across the species' range in the United States that include Arizona, California, and Nevada, USA. Fecal material from 185 snakes yielded 104 prey items from 72 individuals, and palpation indicated recent meal ingestion in 36 of 174 individuals. The diet consisted predominantly of mammals (80.8%), in particular terrestrial squirrels (39.4%) and members of the heteromyid rodent genus Chaetodipus (26.9%). Male and female snakes exhibited similar diets and feeding frequency, but an ontogenetic shift occurred from primarily ectothermic (lizards) to endothermic prey (rodents and birds) and more frequent feeding. Consumption of ectotherms vs. endotherms appeared to be similar among biogeographic regions, but snakes from the Tinajas Altas Mountains, Arizona, preyed disproportionately on birds compared with other regions, and snakes from the Sonoran Desert-Arizona Upland region more frequently ingested recent meals than those from the California-Nevada region. Regardless of sex, size, and biogeographic region, snakes consumed fewer prey during the spring breeding season than during summer and autumn. We provide data on the diet of C. pyrrhus for testing further hypotheses regarding potential interspecific competition, dietary changes associated with climate change, foraging tactics associated with bird predation, competing behaviors during breeding season, and venom composition variation.