Tylosaurus pembinensis (Mosasauridae; Tylosaurini), from the middle Campanian Pembina Member of the Pierre Shale, exposed near Morden, Manitoba, Canada, is redescribed and removed from the genus Hainosaurus. In support of our reassignment, we highlight incorrect diagnostic features in the original description that were used as support for the assignment of the new species to Hainosaurus (e.g., small suprastapedial process of the quadrate and the presence of 54 vertebrae in the dorsal and pygal series [33 dorsals and 21 pygals]). In contrast, we note that the fully prepared quadrate possesses key Tylosaurus features such as a moderately large-sized suprastapedial process. We also report that of the original 54 ‘pre-chevron’-bearing vertebrae, that 26 are from a second, unrelated species of mosasaur. Of the 29 remaining vertebrae, it is difficult, due to poor preservation, to identify which of these elements are pygals. Although the exact pre-chevron count is unclear, the preserved remains clearly do not support a diagnosis of a high pygal count consistent with Hainosaurus. In recharacterizing Tylosaurus pembinensis, we discuss a number of problematic characters used to diagnose Hainosaurus, in the context of diagnosing Tylosaurus and not for the purpose of revising the European taxon Hainosaurus bernardi.