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Peptides from oysters have several bioactive functions. In this study, we identified antioxidant peptides from oysters (Crassostrea rivularis) and investigated their structure–function relationship. We used an 8 kDa molecular‐weight (MW) cut‐off membrane and semiprep reversed‐phase liquid chromatography to collect five peptides (F1–F5) and identified the highest‐abundance ion‐peak sequences AWVDY (F1), MSFRFY(F2), EPLRY(F3), RKPPWPP(F4), and YAKRCFR(F5) having MWs of 652, 850, 676, 877, and 943 Da, respectively, using ultra‐performance liquid chromatography‐quadrupole/time‐of‐flight tandem mass spectrometry. These peptides exhibited high antioxidant activities, similar to butylated hydroxytoluene, reduced glutathione, and ascorbic acid. F5 demonstrated the highest scavenging activity for DPPH radicals (IC50 = 21.75 μg/ml), hydroxyl radicals (IC50 = 18.75 μg/ml), and superoxide radicals (IC50 = 11.00 μg/ml), while F3 demonstrated the highest reducing power. Furthermore, F5 significantly protected Caco‐2 cells from H2O2‐induced oxidative damage. These results suggest that the antioxidant peptide F5 is a promising food additive that protects against oxidative damage.