Fucoidans are sulfated fucosylated polymers from brown algae cell walls. We assessed the inhibitory effects of Costaria costata fucoidan on UVB-induced MMP-1 promoter, mRNA, and protein expression in vitro using the immortalized human keratinocyte (HaCaT) cell line. Pretreatment with fucoidan significantly inhibited MMP-1 protein expression compared to UVB irradiation alone. Fucoidan significantly reduced MMP-1 mRNA expression and inhibited UVB-induced MMP-1 promoter activity by 37.3%, 53.3%, and 58.5% at 0.01, 0.1, and 1 microg/mL, respectively, compared to UVB irradiation alone. C. costata fucoidan may be a potential therapeutic agent to prevent and treat skin photoaging.