Redescription of the Upper Cretaceous marine squamate Acteosaurus tommasinii von Meyer, 1860, notes at least 10 cervical vertebrae, 27 dorsal vertebrae, an absence of pachyostosis on the vertebrae and ribs, overall limb reduction where the forelimbs are smaller than the hind limbs, a flattened pubic bone with a square pubic process and subrectangular descending ramus, small postzygapophyses on the trunk vertebrae, long and narrow neural spines on the caudal vertebrae, and posteroventrally positioned, unfused haemal arches on the caudals. Cladistic analysis of Acteosaurus and other marine squamates, including mosasauroids, dolichosaurs, pontosaurs, Adriosaurus, and Aphanizocnemus, and living and fossil snakes (Dinilysia, Yurlunggur, Eupodophis, Haasiophis, Pachyrhachis, Scolecophidia, Macrostomata, and Anilioidea) resulted in three most-parsimonious trees of 280 steps (C.I. = 0.59, R.I. = 0.81, R.C. = 0.48). All recovered tree topologies found support for the monophyly of the clade Ophidiomorpha, inclusive of Dolichosauridae (Dolichosaurus, Coniasaurus), Pontosaurus, Aphanizocnemus, Adriosaurus, Acteosaurus, and Ophidia. Acteosaurus is found to be a basal ophidiomorph and the sister taxon to the Ophidia.