Regnellidium is a monotypic genus of ferns of family Marsileaceae.

[1] 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).

A fossil assigned to the species Regnellidium upatoiensis has been found in Cretaceous deposits of the eastern United States.

[2] The genus name of Regnellidium is in honour of Anders Fredrik Regnell (1807–1884), a Swedish physician and botanist.