Fucoidan is a heterogeneous group of sulfated, fucose rich polysaccharides predominantly isolated from brown seaweeds, including Fucus vesiculosus, Undaria pinnatifida, Cladosiphon okamuranus, Sargassum spp., and Laminaria spp. During the last several decades, fucoidan has attracted considerable interest in biomedical research because of its broad range of biological activities, including anti inflammatory, immunomodulatory, anticoagulant, antithrombotic, antioxidant, antiviral, cardioprotective, metabolic, and anticancer effects. Importantly, fucoidan should not be regarded as a single chemically defined active pharmaceutical ingredient. Rather, the term describes a structurally diverse family of sulfated polysaccharides whose biological properties vary according to algal source, extraction and purification procedures, molecular weight, monosaccharide composition, degree and pattern of sulfation, and glycosidic linkage. This structural heterogeneity represents one of the principal challenges in translating experimental findings into clinical applications. Preclinical investigations have demonstrated that fucoidan can modulate several signaling pathways involved in inflammation, tumor growth, apoptosis, angiogenesis, metastasis, oxidative stress, immune responses, and coagulation. In oncology, experimental evidence is particularly extensive, with animal and cell based studies suggesting inhibitory effects on tumor proliferation, angiogenesis, invasion, and metastatic dissemination. However, clinical evidence remains considerably more limited. A double blind randomized controlled trial in 54 patients with metastatic colorectal cancer found a significantly higher disease control rate when low molecular weight fucoidan was administered as a supplement to chemotherapy and targeted therapy, but no significant improvement in overall survival, progression free survival, objective response rate, quality of life, or adverse event outcomes was demonstrated. A systematic review of fucoidan as supplemental therapy in cancer patients identified only four eligible clinical studies involving 118 patients and concluded that the clinical evidence remained inconsistent and was limited by small sample sizes and methodological heterogeneity. Human studies investigating immunomodulatory effects, osteoarthritis, metabolic disease, and liver dysfunction have similarly produced preliminary but insufficient evidence for routine therapeutic use. Fucoidan appears to be generally well tolerated in short term human studies, although potential anticoagulant effects warrant caution in patients receiving anticoagulant or antiplatelet medications. Contemporary research increasingly focuses not only on fucoidan as a direct therapeutic agent but also on its incorporation into nanoparticles, hydrogels, films, and other drug delivery platforms. Thus, as of 2026, fucoidan should be considered a promising marine derived bioactive polymer and investigational therapeutic platform rather than an established treatment for cancer or other major diseases. Future clinical development will require rigorous structural characterization, pharmaceutical standardization, pharmacokinetic studies, well designed randomized controlled trials, and clear differentiation between specific fucoidan preparations. Fucoidan, phaeophyta, cancer, inflammation, pharmacology, clinical trials. 1358-1367 Issue-4 Volume-10 Latife Ceyda Irkin