Sulfated polysaccharide PF2 was isolated from the green seaweed Monostroma angicava. PF2 is a rhamnan-type sulfated polysaccharide with specific rhamnopyranose units. PF2 possesses a high anticoagulant activity in vitro. PF2 is a strong thrombin inhibitor mediated by heparin cofactor II. PF2 could be a potential anticoagulant polysaccharide.