Most spiders are exclusively terrestrial with only some groups venturing into aquatic habitats, and only a handful of representatives colonizing the marine intertidal zone. Here, we describe a new intertidal trapdoor spider species, Idioctis parilarilao sp. nov., from southern Taiwan and Green Island (Lutao). The original DNA barcode enables species delimitation between different populations from Taiwan and the morphologically closest species, Idioctis xmas, from Christmas Island, Indian Ocean. These analyses reveal that all populations from Taiwan, including Green Island, are conspecific, and are distinct from Idioctis xmas. Given the lack of representation of other species in public databases, our COI gene tree should be seen as preliminary, but it does support the monophyly of the genus Idioctis. As the genus was not previously known from East Asia, our discovery vastly extends the distribution range of Idioctis and the family Barychelidae. Given the tectonic and volcanic specifics of the history of Taiwan and Green Island, and the direction of the known oceanic currents, we provide a preliminary explanation of the biogeographic and diversification history of intertidal spiders in the region.