Quinoa may be the best plant-based source of iron available. One cup of quinoa nearly meets a day’s requirement for iron, as does its alternative grain friends amaranth, teff, and perhaps even kaniwa. (See more on iron rich food, particularly iron in quinoa).
However both quinoa and amaranth (and all grains) contain a substance called phytic acid (phytates) that binds to iron and reduces your body’s ability to absorb the iron.
If you need to rely on quinoa for your iron intake, I encourage you to read more about soaking grains to improve your iron absorption (or read about grains and phytic acid). If that information interests you, note that there is an entire 40+ page paper you can purchase, packed with graphs from the food science literature and recipes you can use in your own kitchen.
If you eat meat, have sauteed clam over a bed of soaked and cooked quinoa. Drizzle olive oil over top and you will be in iron/olive oil heaven. Clam is the food highest in iron.
Quinoa is not the only iron stand-out in the grain world. Other grains from across the globe have been introduced in the American market and do provide iron, as do some usual suspects. You can read more about these iron rich foods including iron in teff, spelt, whole wheat, amaranth, and buckwheat. These are all great sources of iron which can be improved even more by unlocking the iron in the grains with the strategies outlined in the phytic acid white paper.