Mollusk species associated with Cystoseira barbata were examined in the Sea of Marmara in 2012 at 14 sampling sites with a depth ranging from 0.2 to 0.5 m. A total of 18 468 specimens belonging to three classes (Polyplacophora, Gastropoda and Bivalvia) and 30 species were identified. Alvania mamillata Risso, 1826 is a new record for the marine mollusk fauna of the Sea of Marmara. The number of species at the sampling sites varied from 2 to 11 (400 cm–2), the density ranged between 758 and 63 083 ind. m–2, and wet weight between 0.4 and 34.3 g m–2. Gastropoda were the richest class, represented by 22 species and 7611 individuals. The families Rissoidae, Mytilidae and Pyramidellidae were represented by the largest number of species. The most dominant mollusk species in C. barbata facies were Mytilaster minimus (Poli, 1795) (37.77%), Rissoa splendida Eichwald, 1830 (27.20%), Mytilaster lineatus (Gmelin, 1791) (18.69%) and Bittium reticulatum (da Costa, 1778) (6.89%). However, the mollusks R. splendida (86%), M. lineatus (71%), M. minimus (69%) and Steromphala adansonii (Payraudeau, 1826) (67%) were characterized by the highest values of the frequency index. Canonical correspondence analysis showed that the main factors affecting the assemblages of mollusks were the content of total nitrogen, the maximum thallus height, algal wet weight and temperature.