The stable carbon-isotope composition of mammalian tooth enamel is a powerful tool for reconstructing paleodiet and paleoenvironment. Its application in the fossil record relies on a thorough understanding of the isotopic composition of mammalian diets in modern ecosystems. We compiled and evaluated a global dataset of the carbon-isotope values of artiodactyl tooth enamel, supplemented by new samples, for 79 extant species. After correcting for differences in atmospheric carbon-isotope composition, body mass, and digestive physiology, we compared the inferred carbon-isotope values of ingested forage (δ¹³Cdiet) among seven feeding categories. The artiodactyl herbivore dietary spectrum is expressed through a wide range of δ13Cdiet values, with the most depleted mean value in frugivores and the most enriched in obligate grazers. In general, grazing species have a broader range of isotope values than browsing species, suggesting a wider dietary niche breadth. Notably, variable grazers exhibit a bimodal distribution of δ¹³Cdiet values, with North American and Asian taxa consuming C₃ diets and African taxa consuming C₄ diets, reflecting the amount of C₄ vegetation in the environment. Variation in δ¹³Cdiet values also occurs among terrestrial ecoregions and artiodactyl clades. Grassland ecoregions differ significantly from forest ecoregions. We detected a low but significant phylogenetic signal in the mean δ¹³Cdiet values of extant species, with some of the oldest ruminant lineages having maintained C₃ feeding and pure C₄ diets being restricted to two bovid clades. Determining variation in δ¹³Cdiet values in different feeding categories and lineages will help refine paleoecological and paleoenvironmental reconstructions from the rich fossil record of artiodactyls.