Identical for genus and species due to monotypy. Umoonasaurus possesses the unique-derived features (within Plesiosauria) of thin, arching crests on the midline of the snout (an elaboration of the midline ridge) and above the orbits, a large triangular pineal opening bordered by high fluted, crest-like margins. It further differs from all other plesiosaurs in displaying a distinctive mosaic of primitive and derived features (see character matrix in electronic supplementary material).

Umoona (Antakirinja) indigenous name for the Coober Pedy area, and sauros (Greek), lizard; demos (Greek), of the people, and scyll (Greek), a sea monster of classical mythology—referring to the type locality and purchase of the holotype specimen by public donations.