Giant multilocular prostatic cystadenoma is a rare benign tumor that originates from the prostate gland. It usually is between the rectum and the bladder, and is composed of predominantly cystic enlarged prostatic glands in a fibrous stroma and spreads extensively in the pelvis. The mass usually causes a series of obstructive symptoms by compressing adjacent organs. Here we report a case of a giant multilocular prostatic cystadenoma in a 16-year-old patient, who is the youngest case reported up to now, complaining of difficulty in defecation.