Amaranthus pedersenianus N. Bayón & C. Peláez (Amaranthaceae) is a new Argentinian species of Amaranthus L. subg. Albersia (Kunth) Gren. & Godr. and is described and illustrated from the province of Salta. It is found in Andean puna and prepuna in both Salta and Jujuy provinces at altitudes between 2800 and 3775 m. Amaranthus pedersenianus is similar to A. cardenasianus Hunz., A. hunzikeri N. Bayón, A. kloosianus Hunz., A. squamulatus (Andersson) B. L. Rob., and A. urceolatus Benth. by its urceolate pistillate flowers with five sepals. The new taxon differs by its ascendant or erect habit, height of 5–20 cm, presence of leafless, short terminal inflorescences, flowers with usually four or five stamens, and dehiscent fruits.