Abstract A new polysaccharide composed of galactan sulfate with a β-(1→3)-glycosidic linkage has been isolated from the marine clam species Meretrix petechialis. The polysaccharide was homogeneous in its composition containing d-galactose. The glycosidic linkage was examined by 2D DQF-COSY and 2D NOESY spectroscopy. The coupling constant of anomeric proton was 7.8 Hz, suggesting a β-galacto configuration. The downfield shift of H-2 of galactose residue demonstrated the presence of 2- O-sulfonate group. TQF-COSY confirmed that the C-6 position was substituted with a sulfonate group. The anti-HIV activity of the polysaccharides has been evaluated by the inhibition of syncytia formation. The fusion index and percentage fusion inhibition of sulfated galactan were 0.34 and 56% at 200 μg/mL.