Fucoidan was isolated from Vietnamese brown seaweed Sargassum serratum and its structural characteristics were analyzed. Through chemical analysis, IR, and NMR spectroscopy, the structural features of S. serratum fucoidan were determined. The study identified S. serratum fucoidan as belonging to the galactofucan sulfate group. The primary constituents are α-L-fucose, β-D-galactose, xylose, mannose, rhamnose, and glucose, with a molar ratio of 1 : 0.12 : 0.045 : 0.003 : 0.002. The main substituent is sulfate, with a very small amount of acetyl groups. Sulfate groups are located at C4 and C2 positions in the fucosyl and galactosyl residues. The glycosidic bonds in the main chain of fucoidan are predominantly (1→3) linkages, with branch chain (1→2) linkages.