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Antioxidant peptides were derived from the edible part of Indian squid, Loligo duvauceli, via the use of gastrointestinal enzymes for up to 12 hours. The active α-chymotrypsin hydrolysate exhibited the maximum activity compared to the trypsin and pepsin hydrolysates, as assessed by various radical scavenging and metal chelating assays. Moreover, essential and non-essential amino acids were present in the active hydrolysate. Furthermore, the hydrolysate was purified by ion exchange and gel filtration chromatography using fast protein liquid chromatography (FPLC). The purified peptide exhibited strong free radical scavenging, metal chelation and reducing power abilities that were confirmed with a UV-visible and electron spin resonance (ESR) spectrometer. The peptide also prevented DNA damage and inhibited lipid peroxidation due to its small size (682.5 Da) and content of the high redox potential amino acid sequence Trp-Cys-Thr-Ser-Val-Ser, which was confirmed by ESI-MS/MS. This peptide exhibited no cytotoxic effects on breast cancer cells (MCF7) and scavenged reactive oxygen species at the cellular level under H2O2-induced stress. These results suggest that the peptide derived from squid mantle protein acts as a potent antioxidant against oxidative stress and could be used as an efficient and safer adjunct in food processing.