This review summarizes the structural chemistry and bioactivities of sulfated polysaccharides from marine algae. Marine algae are rich sources of sulfated polysaccharides, including agarans, carrageenans, ulvans, and fucans, which have various biological activities such as anticoagulant, antiviral, immuno-inflammatory, antilipidemic, and antioxidant properties. The review discusses the structural diversity of these polysaccharides, their chemical modifications, and the methods used for their structural analysis, including desulfation, methylation, NMR, and MS. It also highlights the relationship between the structure and biological activity of these polysaccharides, particularly their anticoagulant and antithrombotic activities. The review also covers their antiviral, immuno-inflammatory, and antioxidant activities, as well as their potential therapeutic applications. The study emphasizes the importance of understanding the structural features of these polysaccharides to develop more effective therapeutic agents. The review concludes with a discussion on the future perspectives of algal sulfated polysaccharides in therapeutic applications, including their potential for use in cosmetics and cosmeceuticals.This review summarizes the structural chemistry and bioactivities of sulfated polysaccharides from marine algae. Marine algae are rich sources of sulfated polysaccharides, including agarans, carrageenans, ulvans, and fucans, which have various biological activities such as anticoagulant, antiviral, immuno-inflammatory, antilipidemic, and antioxidant properties. The review discusses the structural diversity of these polysaccharides, their chemical modifications, and the methods used for their structural analysis, including desulfation, methylation, NMR, and MS. It also highlights the relationship between the structure and biological activity of these polysaccharides, particularly their anticoagulant and antithrombotic activities. The review also covers their antiviral, immuno-inflammatory, and antioxidant activities, as well as their potential therapeutic applications. The study emphasizes the importance of understanding the structural features of these polysaccharides to develop more effective therapeutic agents. The review concludes with a discussion on the future perspectives of algal sulfated polysaccharides in therapeutic applications, including their potential for use in cosmetics and cosmeceuticals.