The selenium (Se)-containing imidazole compound selenoneine (2-selenyl- N α , N α , N α -trimethyl- l -histidine) is a strong scavenger of reactive oxygen species (ROS) in the blood and tissues of fish. Intravenous injection of selenoneine into yellowtail has been shown to delay changes in meat color and prevent met-myoglobin formation in red muscle. In this study, to determine whether selenoneine can improve stress tolerance and meat quality in fish, we examined the biological antioxidant functions of selenoneine in fish in vivo. Juvenile amberjack ( Seriola dumerili ) were cultured and fed a diet containing selenoneine for 9 weeks. Total Se and selenoneine concentrations increased in amberjack blood and muscles during the study period. We also measured the oxidative–redox potential (ORP) in fish muscle using an ORP electrode and found that muscle ORP and ROS levels were closely correlated with the Se concentration in blood and muscles. We conclude that dietary administration of selenoneine led to its accumulation in amberjack blood and muscles, resulting in reduced ORP and ROS levels in the muscles.