Syllis Savigny in Lamarck, 1818, the type genus of Syllidae, is the largest and most diverse within the family. This genus presents many taxonomic, biogeographic, and phylogenetic problems due mainly to the lack of molecular data and morphological synapomorphies for many of the species, but also due to poor or inadequate descriptions. In order to improve the knowledge of the genus, we have undertaken a morphological revision of Australian species, based on type material and material collected over years by the Australian Museum. This is the third paper dealing with this family. Eighteen species are herein described, of which five are new and twelve are redescriptions of already known species, with four new combinations: Syllis imajimai, n. sp.; S. narranuk, n. sp.; S. noolinga, n. sp.; S. similisunzima, n. sp.; S. thylacine, n. sp.; S. busseltonensis (Hartmann-Schröder, 1982) n. comb.; S. patriciae (Hartmann-Schröder, 1981) n. comb.; S. pharobroomensis (Hartmann-Schröder, 1979) n. comb., S. pharynxcircunfusata (Hartmann-Schröder, 1979) n. comb, S. armillaris (O. F. Müller, 1771); S. augeneri Haswell, 1920; S. benelihauae (Campoy & Alquézar, 1982); S. hyalina Grube, 1863; S. cf. nigrescens Grube, 1878; S. profunda Cognetti, 1955; S. truncata Haswell, 1920; and S. variegata Grube, 1860.