In our previous study, 8-hydroxydaidzein (8-OHDe) was demonstrated to be a potent and unique suicide substrate of mushroom tyrosinase. In this study, the compound was evaluated for in vitro cellular tyrosinase and melanogenesis inhibitory activities in mouse B16 melanoma cells and for in vivo skin-whitening activity in human volunteers. Tyrosinase activity and melanogenesis in the cell culture incubated with 10 µM of 8-OHDe were decreased to 20.1% and 51.8% of control, respectively, while no obvious cytotoxicity was observed in this concentration. In contrast, a standard tyrosinase inhibitor, kojic acid, showed 69.9% and 71.3% of control in cellular tyrosinase and melanogenesis activity, respectively, at a concentration as high as 100 µM. Hence, 8-OHDe exhibited more than an inhibitory effects on melanin production in B16 cells 10-fold stronger than kojic acid. In addition, when a cream containing 4% 8-OHDe was applied to human skin in an in vivo study, significant increases in the dL*-values were observed after three weeks. Moreover, the increase in the dL*-values after 8-week treatment with 4% 8-OHDe (from -0.57 to 1.94) is stronger than those of 2% 8-OHDe treatment (from 0.26 to 0.94) and 2% ascorbic acid-2-glucoside treatment (from 0.07 to 1.54). From the results of the study, it was concluded that 8-OHDe, the potent suicide substrate of mushroom tyrosinase, has depigmenting activities in both mouse melanoma cells and in human volunteers. Thus, the compound has significant potential for use in cosmetics as a skin-whitening ingredient.