Amaranthus brownii is an annual herb in the family Amaranthaceae. 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.
A. brownii is considered the rarest plant on Nihoa and has not been directly observed on the island since 1983. Past expeditions collected plant samples and seeds, but no specimens have managed to survive ex-situ conservation efforts outside of its native habitat. There are no known plants or seeds from A. brownii in any botanical gardens. Conservation and recovery plans for A. brownii have been proposed by the United States Fish and Wildlife Service (FWS) which administers the island of Nihoa as part of the Hawaiian Islands National Wildlife Refuge in the Papahānaumokuākea Marine National Monument. In 1996, the plant was listed by the FWS as an endangered species. In 2003, the FWS designated the island of Nihoa as a critical habitat for the plant and it was classified as critically endangered on the IUCN Red List.