Abstract Melanin plays an important role protecting skin against ultraviolet light injury. However, increased production and accumulation of melanin results in a large number of skin disorders. Here, we identified hordenine as an active compound from germinated barley (Hordeum vulgare L.) and investigated the effects of hordenine on melanogenesis and its mechanisms of action in human epidermal melanocytes. We measured melanin content, tyrosinase activity, expression of melanogenesis-related proteins, and cAMP production. Melanin content was significantly inhibited by hordenine. The intracellular cAMP level was also reduced by hordenine. In addition, expression of microphthalmia-associated transcription factor (MITF), an upstream transcription factor of tyrosinase as well as tyrosinase, tyrosinase related protein (TRP)-1, and TRP-2, was inhibited by hordenine. Taken together, these results show that hordenine inhibited melanogenesis by suppressing cAMP production, which is involved in the expression of melanogenesis-related proteins and suggest that hordenine may be an effective inhibitor of hyperpigmentation.