The fossil molluscan fauna of Sandelzhausen (Early/Middle Miocene, SE Germany) comprises a total of 44 species, mostly terrestrial pulmonate snails. Herein we present a paleoecological analysis of this fauna based on an actualistic approach and on data on stable isotopes of carbon and oxygen (assessed from specimens of the freshwater lymnaeid snail Galba dupuyiana and the terrestrial clausiliid snail Pseudidyla moersingensis). The paleoecological reconstruction achieved here is in line with previous works, with some novelties and minor modifications. The basal sediment layers point to a swampy area with ponds and/or oxbow lakes (closed system, as indicated by the covariation between oxygen and carbon isotopic signals of G. dupuyiana), prone to seasonal flooding events. This environment would then gradually transition into a perennial lake, as indicated by: the proportion of planorbids, the appearance of aquatic species intolerant to desiccation, and the decoupling of the covariation between oxygen and carbon isotopic signals of G. dupuyiana. The terrestrial habitat would have developed from a more open environment (semi-arid/sub-humid scrubland) to a sub-humid/humid denser forest afterwards. Still, species from drier and more open environments are present throughout all the layers, suggesting that these habitats persisted in the lake’s hinterland. The mean annual temperature, calculated from the oxygen isotopic composition of P. moersingensis, ranges from 18.5 to 20.5 °C, but with no significant trend of change throughout the layers.