The geographic distribution of three intriguing genera of Syllidae (Annelida, Phyllodocida), Alcyonosyllis, Paraopisthosyllis and Megasyllis, is restricted to the Pacific Ocean, Indian Ocean and the Red Sea. In this study new material of several species in all three genera is identified, and the distributions of the species and genera refined; four species of Alcyonosyllis were found to have increased ranges. Additionally, one new species of Paraopisthosyllis is described. Paraopisthosyllis pardus sp. nov. is characterized by having long bidentate bladed-chaetae, segments covered by small papillae, and a colour pattern consisting in dark red-brown antennae and dorsal cirri and several transversal dark red-brown lines per segment. The three genera share several striking morphological characteristics, such as alternation in the arrangement of dorsal cirri, wide segments with secondary annuli and bright, contrasting colour patterns. Alcyonosyllis species are found in association with other organisms, most noticeably anthozoans, whereas members of Paraopisthosyllis and Megasyllis are free living. Molecular information (sequences of DNA) from Alcyonosyllis species (including the type species, A. phili), and the type of Megasyllis (M. corruscans) is included herein for the first time in a phylogenetic analysis. A phylogenetic analysis performed through different methods (Maximum parsimony and Maximum likelihood) using sequences of three genes (18S, 16S and COI) reveals that all the three genera form a monophyletic group within Syllidae, with several synapomorphies, and a common ancestor probably from the Indo-Pacific. Their geographic distribution pattern, the relationships between these genera and the rest of syllids, and the symbiosis in Alcyonosyllis are discussed.