The Upper Cretaceous marine squamate Pontosaurus lesinensis is redescribed. Extensive re-preparation has exposed new details of the skull, mandibles, and postcranial skeleton. Important new characteristics include a unique supraoccipital-parietal articulation, elongation of all postdentary bones, at least twelve cervical vertebrae, hypapophyses with large unfused peduncles, twenty-eight dorsal vertebrae, and a fused scapulocoracoid with a primary coracoid emargination. Cladistic analysis of all anguimorph squamates, including mosasaurs, aigialosaurs, dolichosaurs, snakes, and Pontosaurus, resulted in 3-most parsimonious trees: 334 steps; C.I. 0.78; H.I. 0.42; R.I. 0.81. Both the strict consensus tree and the 50% majority-rule consensus tree found support for the monophyly of the Pythonomorpha, containing mosasaurs, aigialosaurs, dolichosaurs, and snakes (pachyophiids and modern snakes). Pontosaurus is found to be a dolichosaur nested within the clade Pythonomorpha as the sister taxon to Adriosaurus and the Ophidia.