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The Upper Cretaceous phosphates of Morocco preserve one of the world's most diverse assemblages of mosasaurs, reflecting the adaptive radiation of this clade during the Maastrichtian. Herein, we describe a new mosasaur from these deposits. Although the teeth of this specimen resemble those of 'Platecarpus' ptychodon, suggesting referral to this species, we re-examine and ultimately reject the fundamental validity of 'P.' ptychodon due to the non-diagnostic nature of its holotype and original diagnosis. We instead designate the new specimen as the holotype of a new genus and species, Gavialimimus almaghribensis, gen. et sp. nov. G. almaghribensis is characterized by a highly elongate snout, highly retracted nares and large supratemporal fenestrae, among other features. Phylogenetic analysis under multiple parsimony-based methods reveals novel substructure within the subfamily Plioplatecarpinae, consistently recovering a clade uniting the new species with Selmasaurus and the enigmatic Goronyosaurus. Synapomorphies of this clade include a highly constricted parietal, with Selmasaurus and Gavialimimus being further united by a broadly excavated medial surface of the quadrate suprastapedial process. The cranial morphology of G. almaghribensis also provides new insight into several aspects of mosasaur evolution and comparative anatomy, including adaptive radiation and niche partitioning in Moroccan marine palaeoecosystems. [ABSTRACT FROM AUTHOR]