Recently collected materials of a tylosaurine mosasaur from Haumuri Bluff, New Zealand, are assigned to Taniwhasaurus oweni Hector, 1874. Previously described tylosaurine mosasaur specimens from Haumuri Bluff that were assigned to other taxa are re-evaluated here and assigned to T. oweni. Taniwhasaurus is diagnosed as a tylosaurine based on the possession of a premaxillary rostrum and anterior edentulous process of the dentary. However, Taniwhasaurus also exhibits a prefrontal-maxillary contact in which the prefrontal forms a portion of the naris, thus precluding the frontal-maxillary contact characteristic of all other tylosaurines with the possible exception of Hainosaurus bernardi. The distribution of the frontal-maxillary contact in mosasaurs suggests that it may be correlated with snout elongation.