The examination of rock and stone samples collected at 0.2–3 m in Mersin Bay (Levantine coast of Turkey) revealed the presence of 14 specimens belonging to a new species of Syllidae (Polychaeta), Trypanosyllis sanmartini. The species is mainly characterized by having dark reddish-brown dorsal cirri, pale yellowish body colour, weakly bidentate falcigers, long and filiform dorsal cirri, long proventricle and pharynx, and a trepan with ten large teeth (without a pharyngeal mid-dorsal tooth). The posterior-most segments of the holotype and five paratypes had a cluster of stolons. The morphology and reproductive features of the species are explained and discussed.