This study investigated the inhibitory effects of sea mustard on high-fat diet-induced obesity and hyperlipidemia in Sprague-Dawley rats. Sea mustard (Undaria pinnatifida) powder, sea mustard ethanol extract, and sea mustard ethanol-extracted residue were tested. The ethanol extracted residue had the most beneficial anti-hyperlipidemic activity. Alginate in the sea mustard was considered to be the key component. The ethanol-extracted residue of sea mustard also had antioxidant activity, which may be effective in preventing hyperlipidemia by increasing the enzymatic activity of superoxide dismutase, which can remove active oxygen from the bloodstream.