The Louisiana Quillwort (Isoetes louisianensis) is a small, grass-like aquatic plant of the family Isoetaceae . It occurs in only five locations in Washington and St. Tammany Parishes of Louisiana and some spots in southern Mississippi (USA). It is considered to be an endangered species, partly due to its highly restricted range.

The Louisiana Quillwort occurs predominately on sand and gravel bars on small to medium-sized streams. These plants live for periods underwater. They are regularly inundated as much as 50 centimeters (20 inches) following rains, and may be inundated for long periods in wet seasons. Its habitat follows roughly along the Bogue Chitto River .

Distinctive characteristics of this species include brown-spotted sporangial walls and megaspores with highly reticulate ridges, producing a spiny effect. Leaves are up to 16 inches long.

The Louisiana Quillwort was discovered recently, in 1972, and was listed as an endangered species in 1992.