Sulfated polysaccharides found in seaweed have various physiological functions and are attracting attention as functional polysaccharides. Among them, fucoidan, a sulfated polysaccharide found in brown algae, diatoms, sea cucumbers, and sea urchins, is of particular interest. The sugar chain structure of fucoidan varies depending on the species, and even within the same species, it can change depending on the growth stage and environment, resulting in a rich diversity. Therefore, in recent years, fucoidan is also referred to as fucose-containing sulfated polysaccharide (FCSP). On the other hand, the physiological functions of fucoidan are greatly affected by the sulfate content and the sugar composition, and fucoidans with different sugar chain structures may not exhibit the same physiological functions. Therefore, understanding fucoidan structure is essential in the functional research of fucoidan, and research approaches from the two perspectives of “elucidation of functionality” and “elucidation of sugar chain structure” are important.