Marine and anchialine caves are biodiversity reservoirs, harbouring disharmonic faunal communities with high endemism. The study of cave communities is important for understanding the evolutionary history of many taxa; however, our knowledge of cave diversity is highly biased in favor of large-bodied animals, particularly crustaceans. Meiofauna represents an important but often neglected component of cave biodiversity, due to lack of time and expertise for targeted collecting, as well as inadequate taxonomic capacity. Consequently, the significance of meiofauna in cave systems may have been overlooked and so seriously obscuring our understanding of macro-ecological and evolutionary patterns in cave environments. The World Register of marine Cave Species (WoRCS), a Thematic Species Database of WoRMS (www.marinespecies.org/worcs), is here presented as a valuable resource to overcome this problem. The aim of WoRCS is to create a comprehensive taxonomic and ecological database of cave species from worldwide marine and anchialine cave systems, including planktonic and benthic meiofaunal species. The cave-related information is managed by the WoRCS thematic editors in collaboration with the taxonomic editors of WoRMS, who manage the taxonomic content. The database is an open source and includes information on biological, ecological, and occurrence data for all species. Occurrence data are linked to the Gazetteer of the Marine and Anchialine Caves of the World, which is part of the Marine Regions information system and includes geographical and geological information for all studied cave localities. Currently, the database includes approximately 600 meiofaunal species belonging to 21 groups. Most recorded species are amongst hard-bodied meiofaunal groups, such as Podocopida (112 species), Harpacticoida (109 species) and Cyclopoida (96 species). In contrast, few data exist for other groups that are comparatively diverse outside caves, such as Nematoda (41 species) or Platyhelminthes (31 species). The potential taxonomic and geographical biases of our dataset are discussed, along with the different number of cave exclusive species recorded in each group. Our database confirms that further research about cave meiofauna is crucial to an accurate assessment.