Selenium is an essential micronutrient for humans, and seafood is one of the major selenium source in Japan. Recent studies showed that the tissues of tuna, other predatory fish, and whales contain high levels of selenoneine. Selenoneine contains an imidazole ring with a unique selenoketone group and has an antioxidant activity in vitro and in vivo. The dietary intake of selenoneine through fish consumption is thought to be important for enhancing selenium redox functions in tissues and cells. In addition, selenoneine accelerated the excretion and demethylation of methylmercury through the formation of secretory extracellular lysosomal vesicles via the specific organic cation/carnitine transporter-1 (OCTN1). Dietary intake of selenoneine might decrease the formation of hydroxyl and other radicals and accelerate the excretion of peroxides and heavy metals, and thereby inhibit carcinogenesis, lifestyle chronic diseases, and aging.