Sulfated polysaccharides isolated from Sargassum angustifolium and purified to determine their structural characteristics and biological activities. Crude polysaccharides and fractions (F and F) were chiefly composed of neutral sugars (49.4-68.5%) and sulfates (12.5-23.0%) along with small amounts of uronic acids (1.3-13.6%) and proteins (4.1-4.7%). Polysaccharides were mainly constructed of different levels of fucose (23.9-69.9%) and galactose (22.5-29.8%) sugars. Subfractions with molecular weights ranging from 157.2 to 790.8×10g/mol were identified for isolated polysaccharides. Polysaccharides induced RAW264.7 macrophage cells to release noticeable amounts of nitric oxide and cytokines including IL-1β, TNF-α, IL-6, IL-10 and IL-12 through NF-κB and MAPKs signaling pathways. Sulfate esters of fraction F were necessary to its bioactivity and they were located on carbons 2, 4 and 6 of the major sugars. Fraction F was formed of (1→4)- and (1→3)-linked fucose residues branched at C-2 and C-4 as well as (1→6)-linked galactose residues branched at C-3.