Over the years, seaweeds have been highlighted as valuable natural sources of bioactive polysaccharides. Fucoidan, a sulphated polysaccharide mainly from brown seaweeds composed of a fucopyranose backbone and several monosaccharides, have been reported to possess numerous biological activities such as anticancer, antioxidant, immunoregulatory, antiviral, antithrombic, and anti-inflammatory properties thus may confer health benefits to humans. However, the main mechanisms bridging the structural complexity of fucoidans and their biological activity are mostly ignored. This review provides an update on the current knowledge of green extraction methods, purification, and structural analysis of fucoidan. Factors influencing the biological activities of fucoidan are also discussed. Lastly, the review presents concise information about the current application of fucoidans in the food industry, its utilization as delivery systems for nutraceuticals and its prospects in designing functional foods, as well as the toxicological concerns associated with the use of fucoidans. The structural complexity of fucoidans is dependent on the species, source, harvesting time, among other factors. This in turn greatly influences the biological activity of fucoidans, with high degree of sulphation and low molecular weight particularly linked with increased biological activity. Due to the presence of sulphate moieties, fucoidans easily interact with other polymers with far-reaching applications.