A new ceratosaurian theropod dinosaur, Huinculsaurus montesi gen. et sp. nov., is described here. This taxon is based on the last three dorsal vertebrae and the first and second sacral vertebrae found in association at Aguada Grande, Neuquén Province, Argentina. Although fragmentary, Huinculsaurus shows a unique mix of features which differentiates it from all other theropods, including the sympatric abelisaurid Ilokelesia, and is diagnosed by prezygapophyseal articular facets twice longer than wide, anterior centroparapophyseal lamina strongly developed as an extensive lateral lamina in the posterior dorsal vertebrae, pneumatic foramina located ventrally to the postzygodiapophyseal lamina in the posterior dorsal vertebrae, posteriorly tapering postzygapophysis pointed posteriorly, and an accessory lamina bisecting the parapophyseal centrodiapophyseal fossa in the posterior dorsal neural arches. Phylogenetic analyses recovered Huinculsaurus as most closely related to the Late Jurassic Elaphrosaurus than to other Cretaceous abelisauroids, suggesting the persistence of the elaphrosaurine lineage in South America up to the early Late Cretaceous.