Amaranthus is a dicotyledonous pseudocereal and one of the New World's oldest crops, having originated in Mesoamerica, and was a major food crop of the Aztecs who named it Huautli or Xtes. Popularity in the cultivation and consumption of Amaranthus seed in the modern era began in the mid-1970s with the rediscovery and promotion of amaranth due to its superior nutritional attributes as compared to cereal grains. Today, amaranth is still included in the human diet because of its nutraceutical relevance. Amaranth is considered as a "superfood" because it contains high nutraceutical values such as a high-quality protein, unsaturated oils, squalene, dietary fiber, tocopherols, tocotrienols, phenolic compounds, flavonoids, vitamins, and minerals. Compared to other grains, amaranth has a higher amount of protein, dietary fiber, calcium, iron, and magnesium; therefore although it is an ancient crop nowadays, it is considered a millennium crop or superfood with relevant nutraceutical values and its agronomic versatility. This comprehensive chapter is focused on amaranth composition and antioxidant properties and provides several potential medical benefits of its valuable components.