This study aims to investigate the photoprotective properties of a Lomentaria hakodatensis ethanol extract (LHE) against ultraviolet B (UVB) radiationinduced cellular damage in human HaCaT keratinocytes. LHE exhibited scavenging activity against intracellular reactive oxygen species (ROS), which were generated by either hydrogen peroxide ($H_2O_2$) or UVB radiation. Moreover, LHE scavenged superoxide anion generated by the xanthine/xanthine oxidase system and hydroxyl radical generated by the Fenton reaction ($FeSO_4$ + $H_2O_2$). Furthermore, LHE exhibited UVB absorptive properties and attenuated injury to cellular components (e.g., lipids, proteins and DNA), resulting from UVB-induced oxidative stress. In addition, LHE reduced apoptosis in response to UVB, as shown by decreased DNA fragmentation and the formation of apoptotic bodies. These results suggest that LHE protects human keratinocytes against UVB-induced oxidative stress by scavenging ROS and absorbing UVB rays; thereby reducing damage to biological components.