Fragmentary fossils have been tentatively attributed to Alligator from the Eocene of Wyoming, North Dakota, Saskatchewan, and Utah. The earliest definitive temporal and geographic range of Alligator has been limited to the late Eocene and early Oligocene Chadron and Brule Formations of the White River Badlands in South Dakota, represented by fossils of Alligator prenasalis. New fossils from the Chadron Formation in northwestern Nebraska expand the definitive geographic range of Alligator during the late Eocene. Results from a cladistic analysis of 28 alligatoroid species using 181 morphological characters support the identification of the new specimens as representing the most basally divergent taxon within Alligator, suggesting that they likely represent A. prenasalis; however, a lack of diagnostic features in the Nebraska specimens prevents a definitive assignment to this species. Limited occurrence data may indicate that Alligator experienced a reduction in biogeographic range following global cooling around the Eocene-Oligocene transition, although this may be an artifact of low sample size for Oligocene alligators in the Great Plains.