Melanosis in which melanin deposition is found in the epithelia and stroma or blue nevus of the prostate gland is an uncommon lesion, and the histogenesis is still unclear. To our knowledge, there have been only two case reports in which prostatic melanosis and prostatic adenocarcinoma were both present. This article reports a third case having this combination and discusses the existence of melanin pigment in the carcinoma cell with melanosis.