The flesh of the Indian fresh-water mussel, Lamellidens marginalis (LM; Lamarck, 1819), is the byproduct of pearl culture and a cheap protein source. The present study investigated the antioxidant content of this ethnomedicinally cited species to outline its importance in food security and disease prevention. LM was found to be rich in polyphenol antioxidants with good correlation with its reducing capacity. LM also showed a significant free-radical-scavenging activity, H2O2-scavenging activity and Fe-chelating activity. To study the effect of this dietary antioxidant against oxidative stress, we took inflammatory arthritis as a model. LM-treated arthritis rats showed a higher antioxidant defence system with elevated superoxide dismutase, total thiol, glutathione S transferase, glutathione peroxidase, total antioxidant status and catalase concentration of haemolysate. Oxidative stress markers like serum thiobarbituric acid-reacting substances, methyl glyoxal, NO and total oxidant status levels were decreased in LM-treated arthritis rats. Hence, the dietary antioxidants of LM were found to be effective in the prevention of oxidative stress in inflammatory arthritis. In conclusion, LM, the cash-crop byproduct, provides a rare opportunity for income and nutrition, not only by providing cheap and available energy, protein and dietary factors, but also by providing antioxidants effective against chronic inflammatory disease.