The fossil record of Neotropical Lutrinae is very incomplete, with a few specimens of Lontra and three records of Pteronura. The published records of Pteronura correspond to remains of Pteronura sp. from Lujanian beds in Entre Ríos province (Argentina) and P. brasiliensis found in the “Late Pleistocene” of Brazil, but they lack stratigraphic context and their age could range from at least 380–0.9 Ka. P. brasiliensis, the giant river otter, inhabits rivers and lakes of northern central South America, and has been related to the North American fossil otter Satherium. In this work we describe and compare a very complete specimen of Pteronura found in Entre Ríos province using classical morphological descriptive and multivariate analyses, discuss the importance of this specimen for the fossil record of the taxon, and perform a preliminary phylogenetic analysis. The specimen agrees in morphometric and qualitative characters with the recent species P. brasiliensis, but is slightly larger and presents some minor differences in skull and dental morphology. These differences are interpreted as intraspecific variation, thus the fossil is interpreted as P. brasiliensis. In the phylogenetic analysis, the studied specimen clusters with P. brasiliensis, corroborating the taxonomic determination and supporting a relationship between Satherium and Pteronura. The specimen is the first fossil record of the genus for Argentina, and the oldest corroborated record, with a 130–125 Ka tentative age.