The present invention relates to an antioxidant composition and, more specifically, to an antioxidant composition including a Sargassum serratifolium extract or a fraction thereof as an active ingredient, in which the antioxidant composition is a useful material (substance) that can be added to medicines, cosmetics, foods, animal feeds, and the like. The present invention also relates to a pharmaceutical composition for preventing or treating eye diseases, a food composition for alleviating eye diseases, and a health functional food, each of which includes a Sargassum serratifolium extract or a compound separated therefrom as an active ingredient.