The present invention relates to an anticoagulant and a composition for preventing thrombus formation, which contain poly-gamma-glutamic acid (PGA) as an active ingredient. The inventive PGA is a water-soluble, anionic, biodegradable and edible amino acid polymer material, which has an anticoagulant effect of preventing thrombi from being accumulated in blood vessels, shows an excellent sustained-release effect and is harmless to the human body. Thus, it is useful as a high-value-added anticoagulant and a food or beverage composition for preventing thrombus formation.