Lectins, noncatalytic sugar-binding proteins of nonimmune origin, are widely distributed in the most common foods and feeds in varying amounts. Since many lectins, such as legume lectins, are relatively stable against heat denaturation and proteolytic digestion, the digestive tract is constantly exposed to biologically active lectins contained in fresh and processed foods. Lectins interact with the epithelial surface of the intestine and cause adverse effects, sometimes called food poisoning, in humans and animals. Meanwhile, many interesting biological functions have been discovered in lectins originating from foods or foodstuffs, including immunomodulating effects, selective cytotoxicity against cancer cells, antimicrobial and insecticidal activities, modulating effect on the intestinal transport system, and so on. This review aims to present the current state of research on lectins as bioactive proteins in foods and feeds in order to provide opportunities for application development.