from the Star-K website:
Quinoa was determined to be Kosher L’Pesach. It is not related to the chameishes minei dagan-five types of grain products, nor to millet or rice. Quinoa is a member of the “goose foot” family, which includes sugar beets and beet root. The Star-K tested quinoa to see if it would rise. The result was as Chazal termed, sirchon; the quinoa decayed – it did not rise. Furthermore, quinoa does not grow in the vicinity of chometz, nor does its growth resemble kitniyos (see Igros Moshe O.C. Vol. 3, 63). Therefore, quinoa is 100% Kosher L’Pesach. It is recommended to purchase from manufacturers that do not process Chometz grains. Consumers are urged to carefully check grains before Pesach for extraneous matter.