We investigated the effect of starfish (Asterina pectinifera) polysaccharide on the progression and metastasis of human breast cancer cells. At a concentration range of 10 ~ 120 ${mu}g/mL$ the polysaccharide significantly decreased the expression of cyclooxygenase-2 (COX-2) protein induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) and of aromatase mRNA. In a wound healing assay, motility of human MDA-MB-231 breast cancer cells was prevented by the polysaccharide in a dose-dependent manner. These results indicate that starfish polysaccharide can prevent breast cancer progression and metastasis by decreasing prostaglandin $E_2$ and estrogen biosynthesis by COX-2 and aromatase, and by inhibiting cell motility. This report presents information regarding the effectiveness of starfish polysaccharide as a chemopreventive agent against breast cancer.