Natural polysaccharides made predominantly of sulfated α-L-fucose residues are known as fucoidan. They are present in brown algae and in some marine invertebrates. The amount of fucoidan in brown algae varies from 20% in Fucus to 2% in Laminaria. Fucoidan, over the past few years, has attracted steady attention as a readily accessible biopolymer possessing diverse biological activity. Algal fucoidans have much more complex and heterogeneous structures devoid of regularity. It is noteworthy that fucoidans isolated from species belonging to different orders of brown algae differed in the structure of their main chain that was built of either 3-substituted or alternating 3-and 4-substituted α-L-fucose residues. Algal fucoidans are mainly composed of sulfated fucose. As the type of their glycosidic bond and sulfate group distribution are still under debate, their molecular structure remains to be strongly established. Fucoidan displays diverse biological activity with potential medicinal value, such as being an anticoagulant, antithrombotic, anti-inflammatory, antitumor, contraceptive, antiviral, and antioxidant. This review discusses the different extraction methods, purification methods, the structural features, and activities of fucoidan.