A new aigialosaur is described from the lower Turonian (Upper Cretaceous) of Nuevo León State, Mexico, on the basis of the posterior half of an articulated skeleton. It is differentiated from other aigialosaurs by a unique combination of characters, including an absent ultimate rib preceded by six short ones, a single pygal vertebra, and an especially large tibia. The scales are uniform, smooth, rhomboidal, and arrayed in oblique rows, as in other aigialosaurs where they are known. Abdominal contents include 15 carbonate pebbles. Many of the features of the posterior presacral ribs and anterior caudal vertebrae are widespread in Anguimorpha, but the extreme elongation of the neural spines of the caudal vertebrae appears to unite described aigialosaurs, including the new species, with Mosasauridae. The new species provides further evidence that aigialosaurs may have had a more primitive pes than previously thought, whereas the pes in a number of dolichosaurs shows the loss of one phalanx in pedal digit V. The presence of this new species, the oldest aigialosaur described from North America, on a different continent from previously known aigialosaurs and in rocks representing the outer shelf suggests an underappreciated ability of primitive mosasauroids to move about in the open sea.