Three known bromophenols, 2,3-dibromo-4,5-dihydroxybenzylaldehyde (1), 2,2',3-tribromo-3',4,4',5-tetrahydroxy-6'-hydroxymethyldiphenylmethane (2) and bis(2, 3-dibromo-4,5-dihydroxylbenzyl) ether (3), and one new one, 5,5″-oxybis(methylene)bis (3-bromo-4-(2',3'-dibromo-4',5'-dihydroxylbenzyl)benzene-1,2-diol) (4), were isolated from an extract of the red alga, Vertebrata lanosa. The antioxidant activity of these four bromophenols was examined using one biochemical and two cellular assays: Oxygen Radical Absorbance Capacity (ORAC), Cellular Antioxidant Activity (CAA) and Cellular Lipid Peroxidation Antioxidant Activity (CLPAA) assays. Compound 2 distinguished itself by showing potent activity, having a better antioxidant effect than luteolin in both the CAA and CLPAA assays and of quercetin in the CLPAA assay. Although several bromophenols are known to be potent antioxidants in biochemical assays, this is the first time their cellular antioxidant activity has been demonstrated.