The present contribution displays a taxonomic and systematic revision of the exceptionally rich mollusk fauna of the Middle Miocene Lake Drniš, Southern Croatia. In total, 44 species of freshwater mollusks (36 freshwater gastropods, 8 bivalves) and 5 species of terrestrial gastropods are recorded. The assemblage is dominated by melanopsids and hydrobiids concerning number of species and individuals. Such a composition is typical among the coeval paleolakes of the Dinarides and is indicative of a perennial, moderately shallow setting. The fauna proves the highly endemic character of the Dinaride Lake System: 95.4% of the freshwater species recorded for Lake Drniš only occur within this system. Moreover, the fauna shows a particularly strong relation-ship with that of nearby Lake Sinj (79.5%). Only four freshwater species (9.1%) are endemic to Lake Drniš. The present work ties the fauna to the stratigraphical framework established for the coeval Dinaride lakes Sinj, Kupres, and Gacko, and suggests an age of ca. 15.7–15.0 Ma, classifying the deposits into the early Middle Miocene (early Langhian). Among the Hydrobiidae (Gastropoda: Truncatelloidea) the new genus Kadolskya Neubauer and Harzhauser gen. is introduced. Among the Dreissenidae (Bivalvia: Veneroidea) the new subfamily Congeriinae Mandic and Harzhauser nov. subfam., the new genus Illyricocongeria Mandic nov. gen., and the new species Illyricocongeria moirae Mandic nov. sp. are described.