Abstract. Amaranthus pakai Faccenda & N. Bayón (Amaranthaceae), a new Hawaiian endemic species from the main islands, is described. It is most probably closely related to A. brownii Cristoph. & Caum, an extinct species from Nihoa Island. It also morphologically resembles A. interruptus R. Br. and A. viridis L. Amaranthus pakai differs from these species in its leaves with elliptical lamina, and pistillate flowers with three widely spathulate sepals, which are longer than the fruits. The first collections date back to the 19th century and suggest that this species was formerly rather widespread across the main Hawaiian Islands, but the population probably crashed during the 1800s. However, these early specimens were misidentified until recently. Amaranthus pakai was last seen in the wild in 2014 and, following its assessment against the IUCN criteria, is considered Critically Endangered, as only 30 to 40 plants were seen at that time.