Large multilocular cysts and cystic neoplasms of the prostate are very rare. A healthy 52-year-old man presented with acute urinary retension. Physical examination revealed a large abdominal mass. Pelvic MRI showed a 15x10x9 cm, lobulated, and multiseptated cystic mass in the pelvic cavity. Exploration laparotomy revealed a large cytic mass that compressd and displaced the urinary bladder antero-laterally, and adhered to the prostate and the sigmoid colon. Macroscopically, the mass was grayish white colored, multilocular cysts containing reddish-brown serous fluid. Microscopically, the tumor was composed of glands and cysts lined by the prostatic-type epithelium in the fibrous stroma. The prostatic nature of the lesion was confirmed by the lining epithelium expressing prostate-specific antigen.