Fucoidan, a polysaccharide rich in L-fucose and sulfate ester groups, is derived from brown seaweed and marine invertebrates. Over the past decade, fucoidan has gained significant attention due to its diverse biological activities, including anticoagulant, antithrombotic, antivirus, antitumor, immunomodulatory, anti-inflammatory, lipid-lowering, antioxidant, anticomplementary, hepatoprotective, uropathic, and nephroprotective properties. The structure of fucoidan varies among different species of brown seaweed, with some common components such as fucose and sulfate, but also containing other monosaccharides and uronic acids. The complex structures of fucoidan, including sugar composition, sulfate content and position, molecular weight, linkage mode, and sugar residue sequence, contribute to their varied biological activities. For instance, the anticoagulant activity of fucoidan is influenced by the degree of sulfation, molecular weight, and sugar composition. Fucoidan also exhibits antiviral activity against various viruses, antitumor and immunomodulatory effects, and significant antioxidant properties. Additionally, fucoidan has been shown to reduce blood lipids, inhibit complement activation, and have therapeutic potential in surgery, wound healing, and gastric protection. Despite the complexity of fucoidan structures, ongoing research aims to elucidate the relationships between their structures and bioactivities, which could lead to the development of new drugs and functional foods.Fucoidan, a polysaccharide rich in L-fucose and sulfate ester groups, is derived from brown seaweed and marine invertebrates. Over the past decade, fucoidan has gained significant attention due to its diverse biological activities, including anticoagulant, antithrombotic, antivirus, antitumor, immunomodulatory, anti-inflammatory, lipid-lowering, antioxidant, anticomplementary, hepatoprotective, uropathic, and nephroprotective properties. The structure of fucoidan varies among different species of brown seaweed, with some common components such as fucose and sulfate, but also containing other monosaccharides and uronic acids. The complex structures of fucoidan, including sugar composition, sulfate content and position, molecular weight, linkage mode, and sugar residue sequence, contribute to their varied biological activities. For instance, the anticoagulant activity of fucoidan is influenced by the degree of sulfation, molecular weight, and sugar composition. Fucoidan also exhibits antiviral activity against various viruses, antitumor and immunomodulatory effects, and significant antioxidant properties. Additionally, fucoidan has been shown to reduce blood lipids, inhibit complement activation, and have therapeutic potential in surgery, wound healing, and gastric protection. Despite the complexity of fucoidan structures, ongoing research aims to elucidate the relationships between their structures and bioactivities, which could lead to the development of new drugs and functional foods.