Based on specimens collected in the Mediterranean and Red Sea coasts of Israel, the first lessepsian flatworm (Boninia neotethydis sp. nov.) is described. The genus Boninia includes cotylean polyclads characterized by the presence of multiple accessory (prostatoid) organs. The three species described are distributed in the western Pacific (B. mirabilis) and in the Caribbean (B. antillarum and B. divae). The new species differ from congenerics in the number and arrangement of the prostatoid organs, and in the number of marginal eyes. Specimens of the new species feed on mussels, and their possible effects on Mediterranean mussel cultures should be monitored.