"Amaranthus brownii" had a very limited range; it was found only on the island of Nihoa, located northwest of Kauai. It is thought that this endemic plant had probably always been rare and restricted to Nihoa. Its former habitat is managed by the United States Fish and Wildlife Service and protected as part of the Hawaiian Islands National Wildlife Refuge in the Papahānaumokuākea Marine National Monument. "A. brownii" was one of three endemic and endangered species only found on Nihoa, along with the Nihoa fan palm ("Pritchardia remota") and the Nihoa carnation ("Schiedea verticillata"). At least nine other native plant species can be found in its habitat, including Hawaiian goosefoot ("Chenopodium oahuense"), lovegrass ("Eragrostis variablis"), "koali awa" ("Ipomoea indica"), goat's foot ("Ipomoea pes-caprae" subsp. "brasiliensis"), "Panicum torridum", "naupakas" ("Scaevola sericea"), "Sicyos pachycarpus", "ilima" ("Sida fallax"), and Nelson's horsenettle ("Solanum nelsonii").