Ectopic prostatic tissue detected outside the genitourinary system has been rarely reported. A case with a giant pelvic adenoma that originated from ectopic prostatic tissue is presented. A 71-year male was detected with lower abdominal mass for eight months and recurrent acute urinary retention for one week. This patient already had mild lower urinary tract symptoms for three years. The physical examination, laboratory tests, ultrasonography, and MRI of this patient were analysed, and the pathological diagnosis was ectopic prostatic adenoma. The suprapubic incision for pelvic exploration and tumorectomy was chosen. The recognition and awareness of this unusual lesion is important, in order not to confuse this particular lesion with other pelvic tumors.