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Tyrosinase plays a key role in the production of melanin. A variety of industrial fields have shown interest in the development of tyrosinase inhibitors from plants. In this study, compounds <b>1</b>&#8722;<b>5</b> derived from <i>Leonurus japonicas</i> were evaluated to determine their ability to inhibit tyrosinase. Of these, 10-methoxy-leonurine (<b>1</b>) and leonurine (<b>2</b>) exhibited IC₅₀ values of 7.4 &#177; 0.4 and 12.4 &#177; 0.8 &#956;M, respectively, and acted as competitive inhibitors of tyrosinase, with <i>K</i>i values in the micromolar range. In silico modeling revealed a guanidine group located in the inner cavity and a benzene ring docked within the active site of these compounds. These guanidine pseudoalkaloids show potential not only as tyrosinase inhibitors but also as lead compounds in new scaffolds for the development of novel inhibitors.