The present paper reports results from the study of a wide collection of specimens of the genus Haplosyllis (Polychaeta: Syllidae: Syllinae) obtained mainly from sponges of different Caribbean regions (Barbados, Bahamas, Belize, Bermudas, Colombia and Venezuela). Four new species are herein described and illustrated. H. aplysinicola n. sp., the most common Caribbean species, found only in species of genus Aplysina; H. chaetafusorata n. sp., found inside the sponge Verongula rigida and characterized by having traces of fusion between blade and shaft on mid-body and posterior chaetae; H. navasi n. sp., found in Colombian waters inside the sponge Ircinia strobilina and characterized by having mid-body dorsal cirri of all similar in length and by the broad acicula and proventriculum; and H. niphatesicola n. sp., collected inside sponges of genus Niphates and characterized by alternating long and short mid-body cirri, round dorsal granules throughout, and broad posterior acicula. Finally, H. cephalata and H. gula are registered for the first time in Colombian waters, and new information about their habitat is provided. Current knowledge on the relationships between the symbiotic Haplosyllis and their hosts known to date is discussed.