We developed a new data set of enamel microwear for extant caviomorph rodents (i.e., South American hystricognaths) and inferred the diet of an extinct taxon, Neoreomys australis, from data on microwear. To evaluate frequencies of wear features (pits and scratches) in caviomorphs, we employed low-magnification microwear, which has been used successfully by others to distinguish among the diets of ungulates, primates, and sciurid rodents. We developed 3 broad dietary categories for caviomorphs based on behavioral observations reported in the literature: fruit–leaf, fruit–seed, and grass–leaf. Caviomorphs in general all exhibited wear features indicative of processing hard objects (e.g., seed predation, eating hard fruits, and consuming exogenous grit). Among our grass–leaf group, we identified an exogenous-grit subgroup that included fossorial and dust-bathing taxa. We used a discriminant function analysis of wear features to examine post hoc classification of the caviomorph taxa into the 3 dietary categories. Ours is the 1st study to quantify the distribution of microwear features among modern caviomorph rodents; it has the potential to clarify the diets of modern forms that have little behavioral data as well as to infer the diets of extinct species.