Mesogobius nonultimus, or the Caspian toad goby, is one of the species of gobiid fish endemic to the brackish-waters Caspian Sea (lake). It will grow up to a length of 17.4 cm. It has been recorded from Turkmenistan, Daghestan, Azerbaijan and Iran, and is probably found throughout the lake, in the coastal zone down to 50 m depth. Its sister taxon in the Black Sea basin is Mesogobius batrachocephalus.