Tyrosinase is a key enzyme in the synthesis of melanin and is widely distributed in animals, plants, and microorganisms. As excessive melanin production causes not only hyperpigmenting effects on human skin but also melanosis in various foods, an inhibitor of tyrosinase has become of interest lately from a practical point of view. In the present study, we purified the tyrosinase inhibitor produced by Trichoderma viride strain H1-7 from a marine environment. The purified inhibitor showed a single peak on HPLC. The chemical structure of this compound was determined by NMR and mass spectrometry analyses. The structure was the same as homothallin II that has been isolated as an antibiotic from T. koningii and T. harzianum. The inhibitor showed competitive inhibition against mushroom tyrosinase.