Gnathomortis is an extinct genus of marine lizard belonging to the mosasaur family. Fossils of Gnathomortis have been recovered from the Early Campanian Mancos Shale of Colorado. The genus contains a single species, G. stadtmani, considered a species of the related Prognathodon up until its 2020 redescription. It was a large mosasaur measuring 10.5 metres (34 ft) long.