Erigeron maniopotamicus is a rare species of flowering plant in the aster family known by the common name Mad River fleabane. It is endemic to northwestern California, where it is known from only four locations in Humboldt and Trinity Counties. Erigeron maniopotamicus grows in open areas in forest, woodland, and meadow habitat along the path of the Mad River, generally in barren areas without much plant cover. The soils are rocky and tan in color and occur near areas of serpentine soils, but the plant does not occur on the serpentine soil.