A brief introduction gives the state of the art concerning allergies related to the consumption of seafoods. In this type of product, allergenic substances are mainly muscle proteins and thus largely concern fish, shellfish, mollusks, and crustaceans. The literature available regarding food intolerance caused by eating seaweeds is very restricted. Paradoxically, many works have been carried out on the antiallergic properties of macroalgae. This chapter deals with the species shown to be active against allergies and the specific compounds involved, including phenols, in particular phlorotannins in Phaeophyceae, and polysaccharides, such as alginates and porphyrans. Perspectives for using some seaweeds in the development of functional foods are discussed. The harmlessness of most edible seaweeds is also mentioned.