Four species of the genus Lepidonotus (Annelida, Polynoidae) have been listed in the Mediterranean, including L. carinulatus (Grube, 1869), L. squamatus (Linnaeus, 1758), L. tenuisetosus (Gravier, 1902) and L. clava (Montagu, 1808), although recent studies confirmed only the presence of the two latter. Based on materials collected in the Radès Area (Gulf of Tunis, Western Mediterranean), we provide a characterization of a population of L. tenuisetosus based on key morphological characters and body width/length relationships and conclude that all previously known Mediterranean reports from Israel, Croatia, Greece, Spain and Turkey did not belong to L. tenuisetosus, but either to L. carinulatus , L. clava or to other, non-identified species. Therefore, our finding represents not only the first correct record of L. tenuisetosus for the Tunisian waters and the Western basin, but for the whole Mediterranean Sea, and allows us to discuss on the possible reasons explaining the presence of this and other species of Lepidonotus in the Mediterranean.