A rare late Sarmatian terrestrial gastropod fauna is described from the Gratkorn Basin. This intramontane basin developed during the Miocene at the junction between the Eastern Alps and the Pannonian Basin System. During the mid-Sarmatian, a considerable drop of the relative sea level, probably coinciding with uplift in the Eastern Alps, caused the Paratethys Sea to retreat from marginal basins and embayments. An alluvial plain developed in the Gratkorn Basin, indicated by fluvial gravel and paleosol formation. A moderately diverse gastropod fauna of 17 species inhabited this alluvial plain. The composition corresponds to Sarmatian faunas from southern Germany and the North-Alpine Foreland Basin. Taxonomically, it can clearly be distinguished from superficially similar and ecologically equivalent faunas of the Pannonian. Thus, we document the value of terrestrial gastropods as a biostratigraphic tool to date the lithologically often very similar freshwater systems in Miocene intramontane basins. The clausiliid Pseudidyla martingrossi Harzhauser & Binder and the camaenid Pleurodonte michalkovaci Binder & Harzhauser are introduced as new gastropod species.