Among the various animal groups represented in the macrobenthic fauna of the Cochin Estuary studied, Polychaeta was the dominant group. Thirty species belonging to 25 genera under 17 families are briefly described. Of these 30 species, 17 are new records from the Cochin Estuary. Salinity and nature of substrata are the two main factors which govern the distribution of polychaetes. Spatial and temporal distribution of polychaetes present in the Cochin Estuary are also given along with the species description.