Ever since the first mosasaur restorations were published, these extinct marine reptiles have been pictured with either notched, forked or undivided tongues. Here, we present an overview of existing iconography, a review of the previous literature, and we discuss how best to reconstruct tongue form in mosasaurs. Despite disagreement about their precise phylogenetic position, most authors consider mosasaurs members of the Varanoidea, derived anguimorphans including Helodermatidae, Varanidae, Lanthanotus and probably snakes. All anguimorphans share a diploglossan (two-part) tongue, in which the foretongue is derived and modified into a highly protrusible chemosensor, while the hindtongue is plesiomorphic, retaining well-developed papillae, mucocytes and robust posterior lobes. We suggest that mosasaurs had a diploglossan tongue that remained in a relatively underived state. The form of the tongue would probably have been most like modern Heloderma or Lanthanotus with a protrusible chemosensory foretongue and a plesiomorphic, papillose hindtongue. Such a tongue is consistent with well-developed vomeronasal chemoreception through tongue-flicking, with the retention of the ancestral function of hyolingual food transport and swallowing following jaw-prehension of prey. The presence of paired fenestrae in the palate associated with the vomers, as well as the presence of pterygoid teeth are in accordance with such a tongue form in mosasaurs.