Detailed examination of four sediment fractions between 63–500 μm showed that benthic foraminiferal assemblages (undifferentiated living + dead) collected from 23 surface-sediment samples along the Marmara Sea coast were dominated by smaller (<250-μm) individuals, averaging 95% of the total fauna. Higher foraminiferal density, species diversity, and restricted size frequency of some species within the 63–250-μm fraction indicate that different lower sieve limits will cause a significant variation in the composition of local assemblages. A 125-μm lower size limit eliminates the high dominance of Bolivina variabilis and Cassidulina carinata below 65 m. For the >63-μm fraction, cluster analysis of species with ≥5% relative abundance revealed three main assemblages, which were associated with the general hydrography of the Marmara Sea. The first assemblage (0–35 m), is dominated by Ammonia tepida, Ammonia spp., and Elphidium spp. Their distribution is linked to organic matter and river sediment discharge and Black Sea surface inflow. The second assemblage (28–50 m) is dominated by Bulimina aculeata, Bo. variabilis, Nonionella turgida, and Bulimina elongata. This assemblage is affected by salinity fluctuations due to seasonal vertical mixing. The third assemblage (36–320 m) has the highest density and diversity, and includes C. carinata, Brizalina spathulata, Br. dilatata, Valvulineria bradyana, Globocassidulina subglobosa, Bulimina marginata, B. costata, Discorbinella bertheloti, Melonis barleanum, and Gyroidina umbonata. This reflects the greater environmental stability of the Mediterranean undercurrent.