The fossil assemblages of the Late Cretaceous of North America are dominated by large-bodied dinosaur species. Associated skeletons of small dinosaurs are exceedingly rare, and small (<10 kg) carnivorous theropods have not previously been reported from these beds. Here, we describe a small dromaeosaurid from the 75-million-year-old Dinosaur Park Formation of Alberta, Canada. Hesperonychus elizabethae gen. et sp. nov. is represented by a pelvic girdle from an animal weighing approximately 1,900 g. Despite its size, the pubes and ilia are coossified, indicating that the animal was somatically mature. This is the smallest carnivorous, nonavian dinosaur known from North America. Phylogenetic analysis of Hesperonychus reveals that it is not closely related to previously described North American dromaeosaurids. Instead, Hesperonychus is a member of the dromaeosaurid clade Microraptorinae, a group containing the 4-winged Microraptor and the feathered Sinornithosaurus, both from the Lower Cretaceous Jehol Group of China. Hesperonychus is the youngest known member of this lineage, extending the temporal range of the clade by 45 million years, and it is the first microraptorine known from North America, providing further evidence for an affinity between the dinosaur faunas of North America and Asia. Study of fossil collections from the Dinosaur Park and Oldman formations of Alberta has revealed numerous isolated bones of small, basal dromaeosaurids, which are tentatively referred to Hesperonychus. These fossils suggest that small dromaeosaurids were a significant component of the carnivore community in this Late Cretaceous biota.