A large ichthyosaur from the Los Molles Formation (Early Bajocian) of the Neuquen Basin, Northwestern Patagonia, Argentina, represents a new genus and species, Mollesaurus periallus. The holotype of M. periallus new species represents, along with the type of Chacaicosaurus cayi that was found at the same locality, the only diagnostic ichthyosaur specimens from the Aalenian-Bathonian interval.