We describe Katepensaurus goicoecheai, gen. et sp. nov., a diplodocoid sauropod dinosaur from the Bajo Barreal Formation (Upper Cretaceous: Cenomanian—Turonian) of south-central Chubut Province, central Patagonia, Argentina. The holotypic specimen is a closely associated partial axial skeleton that includes cervical, dorsal, and caudal vertebrae. The dorsal vertebrae of Katepensaurus exhibit the following distinctive characters that we interpret as autapomorphies: (1) internal lamina divides lateral pneumatic fossa of centrum; (2) vertical ridges or crests present on lateral surface of vertebra, overlying neurocentral junction; (3) pair of laminae in parapophyseal centrodiapophyseal fossa; (4) transverse processes perforated by elliptical fenestrae; and (5) well-defined, rounded fossae on lateral aspect of postzygapophyses. Based on the results of previous phylogenetic analyses, we regard the new taxon as a member of Rebbachisauridae; more specifically, it may pertain to Limaysaurinae, a rebbachisaurid subclade that, to date, is definitively known only from southern South America. As currently understood, the rebbachisaurid fossil record suggests that the clade achieved its greatest taxonomic diversity within a few million years of its extinction during the early Late Cretaceous.