We evaluated cycloalliin, a sulfur-containing imino acid found in garlic and onion, for its possible use as an ingredient in cosmetics. The activity of tyrosinase, an enzyme involved in melanin synthesis, was inhibited by cycloalliin in a dose-dependent manner. Cycloalliin was also found to be a mixed-type inhibitor of the monophenolase and diphenolase activities of tyrosinase with K i values of 56.0 mM and 13.6 mM, respectively. In addition, cycloalliin only weakly inhibited the activity of mushroom tyrosinase. Furthermore, cycloalliin significantly reduced α-melanocyte-stimulating hormone-induced melanin levels and both protein and mRNA levels of tyrosinase in B16 mouse melanoma cells at a final concentration of 3.8 μM. Cycloalliin also significantly inhibited the activity of adenylate cyclase, an enzyme involved in the cyclic adenosine monophosphate-signaling pathway during melanogenesis, and consequently mRNA and protein expression of tyrosinase was reduced significantly. These results show that cycloalliin may be useful as a skin-lightening agent in the cosmetics industry.