Regnellidium is a monotypic genus of ferns of family Marsileaceae. The single living species, Regnellidium diphyllum, the two-leaf water fern, is native to southeastern Brazil and adjacent regions of Argentina. It resembles its relatives from the genus Marsilea, but has 2-lobed leaves (rather than 4). This fern is sometimes grown in aquaria. It is the only non-flowering plant that produces latex. A fossil assigned to the species Regnellidium upatoiensis has been found in Cretaceous deposits of the eastern United States.