Scaphites gilli, n. sp., a densely ribbed laterally compressed ammonite is described from Wyoming, Colorado, South Dakota, and Montana, in the United States, and Manitoba and Saskatchewan in Canada. The species is assigned to Hoploscaphites Nowak, 1912, which is herein treated as a subgenus of Scaphites Parkinson, 1811. Scaphites (Hoploscaphites) gilli is closely related to the fine ribbed European scaphites usually referred to as Scaphites roemeri d'Orbigny. In terms of the standard stages of the Upper Cretaceous, S. (H.) gilli occurs in the middle of the upper Campanian. The species first appears in the Western Interior of the United States in the middle of the range zone of Baculites perplexus, and ranges on up through the range zones of Baculites gregoryensis, B. scotti, Didymoceras nebrascense, and D. stevensoni.