A new species of the notoungulate Eopachyrucos is described on the basis of mandibles and lower cheek teeth from the upper Oligocene Fray Bentos Formation of Uruguay. Eopachyrucos ranchoverdensis, sp. nov., is most similar to the pre-Deseadan/post-Mustersan (late Eocene) Eopachyrucos pliciferus from Patagonia, but is much larger and less hypsodont. The new species has a well-developed postmetacristid on p3 similar to that of Notopithecinae but with higher crowned teeth. The Oligocene record of Eopachyrucos greatly extends its chronologic range. A cladistic analysis of Eopachyrucos and other interatheriids supports the hypothesis that Eopachyrucos is the most primitive interatheriine, retaining a number of primitive features also found in advanced Notopithecinae. The Fray Bentos Fauna shares a few typical elements with Deseadan faunas from Patagonia, but an important distinction between the two faunas is the persistence of some archaic taxa in the Fray Bentos Fauna. The overall composition of the Fray Bentos Fauna is more similar to late Oligocene Deseadan faunas of Bolivia, and this may be attributed to temporal (younger than the Deseadan of Patagonia) or ecological factors.