Chametz is any food product made from wheat, barley, rye, oats, spelt, or their derivatives, which has leavened (risen) or fermented. Our sages have determined that flour from any of these five grains that comes in contact with water or moisture will leaven unless fully baked within eighteen minutes.
Quinoa is not one of these grains, nor is it related to any of these grains, and is therefore not chametz.
However, there are two factors which must be considered with regards to consuming quinoa on Passover:
1) Due to the gravity of the prohibition against chametz, the medieval Ashkenazi rabbis forbade the consumption of kitniyot, legumes, on Passover, as they can be confused with forbidden grains. This includes (but is not limited to): rice, corn, soy beans, string beans, peas, lentils, peanuts, mustard, sesame seeds and poppy seeds.
There are some Halachic authorities who consider quinoa to be kitniyot, while others rule that quinoa, a member of the "goose foot" family (which includes beets and spinach), is not included in the prohibition against kitniyot.
2) The processing of quinoa is generally done in the same locations where wheat is processed. There is very high likelihood that the machines are not adequately cleaned between the processing of the wheat products and the quinoa, which would lead to a reasonable risk of chametz traces being transferred to the quinoa.
In light of the above, if you are an Ashkenazi Jew, you should ask your community rabbi whether he considers quinoa to be kitniyot. And whether you are Ashkenazi or Sephardic, any quinoa would require Kosher for Passover certification to ensure that the plant where it was processed was properly cleaned and purged of any traces of grain products before the processing of the quinoa.
Have a happy and kosher Pesach!
Rabbi Baruch S. Davidson