A mucinous adenocarcinoma of the prostate is rate, and a doubtful diagnosis should be verified to determine that the tumor surely does arise from the prostate, since a mucinous adenocarcinoma arising from the gastrointestinal tract is not as rare and often metastasizes to the prostate. We herein report on a case of a mucinous adenocarcinoma of the prostate, the origin of which was proved to be the prostate by immunohistochemical staining for a prostate-specific antigen and prostate-specific acid phosphatase.