The Socotra Archipelago represents a “hot-spot” of fish diversity in the Indian Ocean. A preliminary specis list comprises presently ~800 species. Employing incidence-based models the total richness is extrapolated at 875 species, thus representing the most species-rich of any comparable Arabian ecoregion. The unique coastal fish assemblages are predominantly composed of coral-reef species, in spite of the few biogenic reef frameworks. They are limited by the Indian monsoon climate and temporary ‘pseudo-temperate’ oceanographic conditions related to upwelling systems. The latter also trigger a primary productivity which underpins a pronounced saisonal pattern of the fish assemblages. The ichthyogeographic results challenge established biogeographic and ecoregional schemes, suggesting the Socotra Archipelago as a distinct ecoregion. This best reflects its exceptionally high marine diversity and its position as a distinct biogeographic transition zone between “Arabia” and the Indo-West Pacific.