A NEW HADROSAURID (DINOSAURIA, ORNITHOPODA) FROM ALLEN FORMATION, LA PAMPA PROVINCE, ARGENTINA. New studies of a Hadrosauridae specimen collected in the Allen Formation, Late Campanian/Early Maastrichthian from La Pampa Province, Argentina, allow recognizing a new taxon, Lapampasaurus cholinoi gen. et sp. nov., which is diagnosed by the following combination of characters: anterior cervical vertebrae with lateral foramina on the dorsal side of the diapophyses, scapula with laterally sharp deltoid crest and convex dorsal surface, pedal ungual phalanx longer than wide with superficial grooves and foramina, and shallow, longitudinal ventral ridge. The identification of Lapampasaurus as a member of Hadrosauridae is confirmed by the recognition of several synapomorphies, such as cervical neural arches with zygapophysial peduncles that elevate above the neural canal, long and dorsally arched postzygapophyses, coracoid relatively small when compared with the scapula, with concave anterior margin and well developed bicipital knob, and hoof-shaped pedal ungual phalanx. The record of Lapampasaurus in the Late Cretaceous of Patagonia suggests the presence of a more diverse hadrosaurid fauna than previously recorded.