It is becoming increasingly important to monitor zoobenthic biodiversity in seas that are under industrial and anthropogenic pollution pressure, such as the Black Sea and the Marmara Sea. This study covers crustacean species in the Turkish waters of the Black Sea and the Marmara Sea, both of which are closed systems. Sampling was carried out in July–August 2019 and yielded 32 crustacean species from the Black Sea and 77 species from the Marmara Sea. In the Marmara Sea, two species [Kupellonura mediterranea and Leucon (Macrauloleucon) siphonatus] represent new records for the Turkish waters, and 12 species [Cirolana cranchii, Cumella (Cumella) pygmaea, Cyathura carinata, Cymodoce truncata, Eurydice pulchra, Gammaropsis sophiae, Harpinia truncata, Iphinoe serrata, Iphinoe trispinosa, Liocarcinus pusillus, Nebalia strausi and Synchelidium maculatum] are new to the Marmara Sea. The following species: Gammaropsis palmata, Pontocrates arenarius, and Synchelidium haplocheles are new records for the Black Sea. The order Amphipoda dominated in both seas in terms of the number of species and the number of individuals.