Tyrosinase requires two copper ions at the active site, in order to oxidize phenols to catechols. In this study, the inhibitory effect of the copper-chelating compound, ammonium tetrathiotungstate (ATTT), on the tyrosinase activity was investigated. ATTT was determined to inactivate the activity of mushroom tyrosinase, in a dose-dependent manner. The kinetic substrate reaction revealed that ATTT functions as a kinetically competitive inhibitor in vitro, and that the enzyme-ATTT complex subsequently undergoes a reversible conformational change, resulting in the inactivation of tyrosinase. In human melanin-producing cells, ATTT evidenced a more profound tyrosinase-inhibitory effect than has been seen in the previously identified tyrosinase inhibitors, including kojic acid and hydroquinone. Our results may provide useful information for the development of whitening agent.