The molluscan fauna of Isla del Coco has recently been well documented, but the heterobranch sea slugs, traditionally called “opisthobranchs”, remain poorly known. We report 13 new records, increasing the total to 40 species. Of the 13 newly discovered species, the following species had not been previously recorded along the Pacific Costa Rican mainland: Berthella californica (Dall, 1900), Peltodoris rubra (Bergh, 1905), Dendrodoris albobrunnea Allan, 1933, Doriopsilla cf. spaldingi Valdés & Behrens, 1998, Glaucus cf. marginatus (Reinhardt & Bergh, 1864), and Flabellina sp. Additionally, we report Peltodoris rubra, previously known from the Indo-Pacific, for the first time in the Eastern Tropical Pacific.

Keywords: Heterobranchia, Nudipleura, Sacoglossa, Aplysiomorpha, opisthobranch, biodiversity, new records, Isla del Coco, Cocos Island.