Marine angiosperms are submerged marine plants with a wide global distribution, occurring in temperate and tropical areas where they form meadows that harbour algae and marine fauna that are important for the functioning of the coastal ecosystem. This study is a floristic survey of the seagrasses occurring in the coastal region of the State of Piauí, Brazil. Five species were recorded for two genera in two families; for the Cymodoceaceae: Halodule wrightii Asch., Halodule beaudettei (Hartog) Hartog and Halodule emarginata Hartog; and for the Hydrocharitaceae: Halophila decipiens Ostenf. and Halophila baillonis Asch. ex Dickie. Species descriptions, an identification key, and information on geographic distributions and habitats of the five taxa are presented. All species are native but only Halodule emarginata is endemic to Brazil, and is reported here as a new record for Piauí, extending its range northwest from the coast of the neighbouring state of Ceará.