Fucoidan is a complex sulfated polysaccharide primarily extracted from the cell walls of brown seaweed species, such as Fucus vesiculosus and other Laminariales. It is composed mainly of α-L-fucose monomers linked by glycosidic bonds, with sulfate ester groups, and may include other sugars like galactose, mannose, and glucuronic acid. This compound exhibits a range of biological activities, including anticoagulant, antiviral, anti-inflammatory, and antioxidant properties, making it valuable in pharmaceutical and nutraceutical applications. Fucoidan is studied for its potential in cancer therapy, immune modulation, wound healing, and as a prebiotic for gut health. Its high molecular weight (typically 20,000 to 1,000,000 Da) contributes to its viscosity and gel-forming abilities in aqueous solutions. As a natural polymer, it is water-soluble due to its hydrophilic sulfate groups, but less so in organic solvents. In food science, fucoidan is used as a functional ingredient in supplements and functional foods for its health benefits, with ongoing research into its bioavailability and mechanisms of action. It is generally recognized as safe (GRAS) by regulatory bodies like the FDA for certain applications, though purity and source matter. Fucoidan's environmental role includes being a structural component in marine ecosystems, and it has low toxicity profiles in both human and ecological contexts.