The study of Syllidae (Polychaeta) from the southwest Atlantic Ocean revealed the presence of an unknown species, which is herein described as Haplosyllis loboi, new species. According to its morphology, the new species belongs to the species-complex group of Haplosyllis spongicola (Grube, 1855). However, it is distinguished by the presence of a series of denticles between the upper end of the main fang and the lower end of the subdistal tooth and the shape of the dorsal cirri, which are shorter than the body width and tapered, showing slightly alternating lengths. The description of this new species of Haplosyllis supports the growing evidence that H. spongicola, formerly considered to be a cosmopolitan species, has in fact been used for several distinct species.