Cyperus pennatiformis (formerly Mariscus pennatiformis) is a rare species of sedge known by the common name coastal flatsedge. It is endemic to Hawaii, where it grows on the islands of Maui, Kauai, and Laysan. It is a federally listed endangered species of the United States. On Maui the sedge grows on seaside cliffs where it is washed with ocean spray. The habitat is a type of coastal forest dominated by hala (Pandanus tectorius). On Kauai it can be found in moist forested canyons. On Laysan ssp. bryanii grows in wetlands and sand dunes.