The fresh water clam Corbicula fluminea is currently one of the most economically important aquatic species in China because of its nutritional value and pharmacological activity. In order to explore the potential of C. fluminea as a natural resource of bioactive compounds, a papain-released polysaccharide designated CFPS-2 was isolated. Chemical composition analysis indicated that CFPS-2 contained glucosamine, glucose, galactose, fucose, protein and sulfate groups, with an average molecular weight of about 22 kDa. Furthermore, the antioxidant and antitumor activities, in vitro, of the polysaccharide fractions (crude CFPS and purified CFPS-2) were evaluated. CFPS-2, which exhibited strong antioxidant activities in a dose dependent manner also showed significant inhibitory effects on growth of human gastric cancer cells (SGC7901) and human ovarian carcinoma cells (SKOV3 and A2780). The present results suggest that CFPS-2 could be a potential candidate for the development of novel functional food ingredient.