Fifteen species of Nereididae (Annelida: Polychaeta) belonging to nine genera are reported from the subtidal and intertidal of the Pacific Coast of Costa Rica. A new species, Nereis costaricaensis, is described. Laeonereis brunnea Hartmann-Schröder 1959 is removed from synonomy with L. culveri (Webster 1880) while Perinereis seridentata (Hartmann-Schröder 1959) is moved from the genus Neanthes based upon the presence of barshaped paragnaths on area VI of the proboscis. The presence of Neanthes micromma Harper 1979 is an extension of the range of this species from the Gulf of California and Neanthes roosevelti Hartman 1939 is the first record of this species since its description from the Galapagos. A taxonomic key to these nereidid species is included.
Keywords: Polychaeta, Nereididae, Costa Rica, Tropical, Eastern Pacific