Amaranthus brownii is an annual herb in the Amaranthaceae family. The plant is found only on the small island of Nihoa in the Northwestern Hawaiian Islands, growing on rocky outcrops at altitudes of 120–215 m (394–705 ft). It is one of nine species of Amaranthus in the Hawaiian Islands, but the only endemic Hawaiian species of the genus. It was first discovered during the Tanager Expedition in 1923 by botanist Edward Leonard Caum. A. brownii differs from other Hawaiian species of Amaranthus with its spineless leaf axils, linear leaves, and indehiscent fruits.