Eight cumacean species, five from shallow water (Cumella spinifera, Coricuma nicoyensis, Cyclaspis breedyae, C. dolera, and C. vargasae), and three from deepwater (?Diastylis tenebricosa, Makrokylindrus menziesi and Vemakylindrus costaricanus), have been previously reported from the coastal and marine waters off Costa Rica. A preliminary study of the estuarine and marine Cumacea of Costa Rica conducted between 1998 and 2003 resulted in the collection of 14 species from Caribbean and 13 species from Pacific. Three families (Bodotriidae, Nannastacidae and Diastylidae) and ten genera were represented in these collections. The species from the Caribbean coasts are represented by the genera Cyclaspis, Vaunthompsonia, Cumella, Elassocumella, and Schizotrema and those from the Pacific by the genera Coricuma, Cyclaspis, Leptocuma, Cumella and Diastylis. Unexpectedly, eight species (C. pustulata, C. jonesi, C. garrityi, C. gomoiui, C. medeeae, C. ocellata, C. ruetzleri, and C. somersi), all of which were previously known from the western Atlantic, are tentatively reported from both coasts of Costa Rica. These apparent new Pacific records for these Atlantic species, especially those for C. pustulata and C. ocellata, require additional study (e.g., DNA fingerprinting) and careful morphological comparison with type material before any definitive taxonomic conclusions can be made. The 25 species (8 records from previous studies and the 17 new records established in the present study) now known from Costa Rican waters are listed systematically in tabular form with ecological information.