THE Late Cretaceous rocks of Mongolia have produced unusual and phylogenetically important dinosaurs1,2. Here we report a startling new example, Mononychus olecranus gen. et sp. nov., an avialian theropod dinosaur with a short, robust forelimb possessing a single stout claw. Several features, including a carinate sternum and reduced fibula, suggest that Mononychus olecranus is more closely related to modern birds than is Archaeopteryx lithographica. The two skeletons are among the best preserved fossils known of a primitive bird, and emphasize the complexity of the morphological transformation from nonavialian theropods to modern birds. The occurrence of such a primitive bird in the Late Cretaceous reflects the paucity of Mesozoic bird fossils and suggests that the early radiation of avialians is only beginning to be sampled.