Abstract A new syllid species belonging to the genus Sphaerosyllis ( Claparède, 1863 ) was found in Southeast Brazil during the development of the project “Environmental, social and economic susceptibility of Ilha Grande Bay to spillage and the presence of oil at sea.” Hence, this paper contains its morphological description, obtained after careful examination of several specimens under light and scanning electron microscopy. The new species can be diagnosed by the presence of intersegmental brown stripes along all body segments, fused palps for most of its length, a prostomium semicircular with three antennae, one simple dorsal chaeta, one simple ventral chaeta and 3–4 falcigers bearing unidentate blades. Finally, an identification key and a comparative table are provided for all the species formally recorded in Brazil to date. Claparède 186 Environmental, Environmental “Environmental sea. sea Hence description microscopy segments length antennae 34 3 4 3– blades Finally date 18 1