A list of freshwater and brackish-water molluscs of Costa Rica includes 50 species (three introduced). They are in the families Unionidae, Mycetopodidae, Corbiculidae, Sphaeriidae, Dreissenidae, Teredinidae (all: Pelecypoda), and in Gastropoda: Neritidae, Thiaridae, Pilidae, Hydrobiidae, Physidae, Lymnacidae, Planorbidae and Bulinidae. A short historical review of malacological collections in Costa Rica is included.