Macroalgae are traditionally used in human and animal nutrition. Several species belonging to the red seaweed family are often described as containing high levels of proteins. In Porphyra tenera, well known under the name of Nori, the protein content is between 33% and 47% of the dry weight (DW). Other red seaweed species such as Palmaria palmata (Dulse) consumed in Europe as sea vegetable also has a high protein level (e.g., 25–35% of DW). Except for red seaweeds, some green algae also possess high protein content. For instance, the algae belonging to the genus Ulva have a protein level that can reach upto 33% of DW. The species Ulva lactuca, called sea lettuce and consumed frequently around the world, has this characteristic. In the past, some of these seaweed species were also used as farm animal feed. Many red and green algae have a protein content comparable to that of soybean, which is the main protein source for animal feeding. Concerning human nutrition, some algal proteins contain a high level of methionine which is an amino acid mainly obtained by the consumption of animal protein. In this context, the use of seaweed as a protein and amino acid source for animal feeding and for human nutrition as well as adapted to the request of vegan consumers is worth discussing. This chapter describes the main seaweed species consumed as sea vegetables and their valorization opportunities as source proteins or amino acids. The main factors limiting the nutritional value of these proteins are also detailed. The low allergenic nature of algae and in particular of their proteins is also discussed.