The first described genus of mosasaur, Mosasaurus hoffmannii, has been coarsely diagnosed and defined since its creation, with numerous specimens and new species being assigned to the genus with little or no reference to, and thus few real similarities with, the generic type specimen. One of the earliest examples of a weakly defined and diagnosed species assigned to the genus is Mosasaurus gracilis. Location and examination of the various assigned specimens and the holotype indicate that M. gracilis shares more characters, such as a short rostrum on the dentary anterior to the first teeth, with russellosaurine mosasaurs. In addition, M. gracilis is known from Turonian deposits, whereas other species belonging to Mosasaurus are upper Campanian—Maastrichtian in age. Based on the evidence of shared characters and contemporaneity, we suggest that M. gracilis be removed from Mosasaurus because it shares more affinities with members of the Russellosaurina. This represents the first step in the very necessary process of untangling the alpha taxonomy, and subsequently the complete systematics, of the genus Mosasaurus hoffmannii.