Photo-protective properties of Polysiphonia morrowii ethanol extract (PME) against ultraviolet B (UVB) radiation-induced cellular damage were investigated in human HaCaT keratinocytes. PME exhibited scavenging activity against 1,1-diphenyl-2-picrylhydrazyl radical and intracellular reactive oxygen species induced by either hydrogen peroxide ($H_2O_2$) or UVB radiation. In addition, PME scavenged superoxide anion generated by xanthine/xanthine oxidase system and hydroxyl radical generated by the Fenton reaction ($FeSO_4+H_2O_2$), both of which were detected using electron spin resonance spectrometry. Moreover, PME, which scavenges $H_2O_2$, was shown to contain the antioxidant compound 3-bromo-4,5-dihydroxylbenzaldehyde, possibly accounting for scavenging actions. PME also restored the levels of catalase and superoxide dismutase protein expression and activity that were decreased by UVB radiation. PME reduced UVB-induced apoptosis, as shown by decreased apoptotic bodies and DNA fragmentation. These results indicate that PME protects human keratinocytes against oxidative stress induced by UVB radiation through the enhancement of antioxidant systems.