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 ...