The paper reviews the structural chemistry and bioactivities of sulfated polysaccharides derived from marine macroalgae. It highlights the structural diversity of these polysaccharides, including galactans (such as agarans and carrageenans), ulvans, and fucans. The review also discusses the anticoagulant/antithrombotic, antiviral, immuno-inflammatory, antilipidemic, and antioxidant activities of these polysaccharides, along with their potential therapeutic applications. The structural analysis methods, such as desulfation, methylation, NMR, and MS, are described, and the impact of molecular size modification on bioactivity is discussed. The paper emphasizes the complex interplay of structural features, including sulfation level, distribution of sulfate groups, molecular weight, sugar residue composition, and stereochemistry, in determining the biological activities of these polysaccharides. Finally, the authors explore future perspectives on the therapeutic potential of algal sulfated polysaccharides, considering challenges such as structural variability and bioavailability.The paper reviews the structural chemistry and bioactivities of sulfated polysaccharides derived from marine macroalgae. It highlights the structural diversity of these polysaccharides, including galactans (such as agarans and carrageenans), ulvans, and fucans. The review also discusses the anticoagulant/antithrombotic, antiviral, immuno-inflammatory, antilipidemic, and antioxidant activities of these polysaccharides, along with their potential therapeutic applications. The structural analysis methods, such as desulfation, methylation, NMR, and MS, are described, and the impact of molecular size modification on bioactivity is discussed. The paper emphasizes the complex interplay of structural features, including sulfation level, distribution of sulfate groups, molecular weight, sugar residue composition, and stereochemistry, in determining the biological activities of these polysaccharides. Finally, the authors explore future perspectives on the therapeutic potential of algal sulfated polysaccharides, considering challenges such as structural variability and bioavailability.