Cossura chilensis is one of the most abundant polychaete species on soft-bottom benthic communities in central Chile. Nevertheless, it was firstly described by Hartmann-Schröder in 1965 and re-described by Carrasco in 1977; both studies with incomplete specimens. The aim of this research was to made a new description of the species in a detailed way from entire specimens using Scanning Electronic Microscopy. Cossura chilensis has a conical prostomium with a dorsal furrow and its peristomium is short and split by transverse ring. The first chaetiger is uniramous and the following ones are biramous. The branchial filament arises from the anterodorsal part of third chaetiger. Chaetae include short capillaries in the 3 first chaetigers; long capillaries in both rami along the rest of the body and thick capillaries in neuropodia of chaetigers 2-10. In addition, short spine-like chaetae are present from posterior segments. All chaetae are hirsute presenting a dense feather-appearance just by one side of the chaetae. Thoracic segments are short and crowded, while abdominal segments are longer than broad and less crowded. The pygidium is small and round-tip with 3 long cirri around anus. Methyl green staining is intense in prostomium, less intense in peristomium, first two chaetigers and in lateral patches from third chaetiger. The highest mean abundance by field work was 897 ind. m-2 in September 2013 being a dominant species of the soft-bottom Valparaiso bay, central Chile.