Rinconsaurus caudamirus gen. et sp. nov. (Dinosauria, Sauropoda) is a new and slender Titanosauridae. The specimens come from the Upper Cretaceous strata of the Río Neuquén Formation at Rincón de los Sauces, Neuquén Province, Patagonia, Argentina. The remains include 13 articulated caudal vertebrae and disarticulated cranial, cervical, dorsal and appendicular materials. These fossils belong to three individuals, two adults and one juvenile. This new species is characterized by the following association of autapomorphies: (1) neural spines in mid-anterior dorsal vertebrae inclined posteriorly more than 60 degrees with respect to the vertical, (2) middle caudal vertebrae with bony processes that support the articular surfaces of postzygapophyses, and (3) procoelous posterior caudal centra with intercalation of a series of amphicoelous-biconvex or amphicoelous-opisthocoelous-biconvex centra. A cladistic phylogenetic analysis placed Rinconsaurus in the family Titanosauridae. Within Titanosauridae (Rinconsaurus + Aeolosaurus) is considered a sister group of the clade (Opisthocoelicaudia + (Alamosaurus + (Neuquensaurus + Saltasaurus))). Rinconsaurus caudamirus has preserved short articulated posterior caudal series with amphicoelous, opisthocoelous and biconvex centra. This unusual morphology represents the first such occurrence in sauropod dinosaurs. From the systematic point of view, this discovery is important because Titanosauridae were traditionally defined, among other characters, by strongly developed procoelia in their caudal vertebrae