Fucoidan is a sulfated polysaccharide mainly derived from brown seaweed, characterized by high fucose and sulfate content. It has been extensively studied for its diverse biological activities, including anticoagulant, antithrombotic, antivirus, antitumor, immunomodulatory, antioxidant, and anti-inflammatory effects. Recent research has elucidated its complex structures, which vary among species and are influenced by sulfate positions, molecular weight, and sugar composition. Fucoidan's structure is often branched, with sulfate groups primarily at C-2 and C-4 of fucose residues. Its biological activities are closely related to its structural features, such as sulfate content and position, molecular weight, and sugar composition. Fucoidan exhibits potent anticoagulant activity, often comparable to heparin, and its anticoagulant effects are mediated by thrombin inhibition and antithrombin activity. It also shows antiviral activity against various viruses, including herpes simplex virus. Fucoidan has antitumor effects by inhibiting tumor cell adhesion and promoting immune responses. It possesses antioxidant properties, reducing oxidative stress and lipid peroxidation. Additionally, fucoidan demonstrates anti-inflammatory effects by modulating inflammatory responses and reducing leucocyte recruitment. It also exhibits gastric protective effects by inhibiting Helicobacter pylori adhesion and reducing ulcer formation. Fucoidan has potential therapeutic applications in surgery, wound healing, and the prevention of transplant arteriosclerosis. Its structural complexity and diverse biological activities make it a promising candidate for drug development and functional food applications.Fucoidan is a sulfated polysaccharide mainly derived from brown seaweed, characterized by high fucose and sulfate content. It has been extensively studied for its diverse biological activities, including anticoagulant, antithrombotic, antivirus, antitumor, immunomodulatory, antioxidant, and anti-inflammatory effects. Recent research has elucidated its complex structures, which vary among species and are influenced by sulfate positions, molecular weight, and sugar composition. Fucoidan's structure is often branched, with sulfate groups primarily at C-2 and C-4 of fucose residues. Its biological activities are closely related to its structural features, such as sulfate content and position, molecular weight, and sugar composition. Fucoidan exhibits potent anticoagulant activity, often comparable to heparin, and its anticoagulant effects are mediated by thrombin inhibition and antithrombin activity. It also shows antiviral activity against various viruses, including herpes simplex virus. Fucoidan has antitumor effects by inhibiting tumor cell adhesion and promoting immune responses. It possesses antioxidant properties, reducing oxidative stress and lipid peroxidation. Additionally, fucoidan demonstrates anti-inflammatory effects by modulating inflammatory responses and reducing leucocyte recruitment. It also exhibits gastric protective effects by inhibiting Helicobacter pylori adhesion and reducing ulcer formation. Fucoidan has potential therapeutic applications in surgery, wound healing, and the prevention of transplant arteriosclerosis. Its structural complexity and diverse biological activities make it a promising candidate for drug development and functional food applications.