The new corticolous lichen fungus Platythecium seychellense is described from the Seychelles. Morphological characters as well as distribution and resemblance to related species are discussed. The species is characterized by a crustose, grey-green, smooth thallus lacking lichen substances, elongate and slender apothecia having flat, red-brown discs and grey 3-septate ascospores. A world key to all currently known species in the genus is presented.