Fucoidans (FU's), sulfated polysaccharides derived predominantly from brown seaweeds, have garnered substantial interest due to their multifunctional biological activities and emerging applications in biomedicine. This review comprehensively examines FU's structural diversity, physicochemical properties, and therapeutic relevance, particularly on native and modified forms. FU content across species ranges from 2 % to 20 % (dry weight), with molecular weights spanning 10–950 kDa and sulfation levels between 5 % and 35 %, all critically influencing bioactivity. Quantitative evaluations reveal that FU's can inhibit tumor cell proliferation with IC50 values ranging from 24 to 72 μg/mL and reduce inflammatory mediators by up to 56 % in in vitro models. Advances in formulation science have enabled the development of FU-based nanoparticles (90–210 nm), hydrogels, microneedles, and scaffolds, achieving up to 87 % encapsulation efficiencies and sustained drug release over 48–72 h. Modified FU's (e.g., oversulfated or conjugated forms) demonstrate enhanced anticoagulant, immunomodulatory, and regenerative effects. This review integrates findings from over two decades (2000–2025), highlighting source-specific properties, bioactivities, and challenges such as low oral bioavailability, structural heterogeneity, and regulatory bottlenecks. Synthesizing structural, pharmacological, and translational insights, this work offers a timely reference that bridges marine-derived biomolecules with next-generation therapeutic platforms.