A 6-year-old boy presented with the chief complaints of miction pain and pollakisuria. He had a past history of acute lymphoblastic leukemia (ALL), which subsided in response to chemotherapy at 3 years of age. Ultrasonography revealed urinary retention associated with bilateral hydronephrosis secondary to the prostate enlargement. Computed tomography and magnetic resonance imaging showed no other abnormal finding. Transrectal needle biopsy showed infiltration of leukemic cells in the prostate. Bone marrow puncture and cerebrospinal fluid aspiration revealed no leukemic cells, resulting in a diagnosis of extramedullary relapse of ALL in the prostate. Although he was successfully treated by chemotherapy, irradiation and his voiding function was improved, ALL relapsed in the left testis 1 year later. In spite of left orchiectomy, irradiation and additional chemotherapy, he died of bone marrow relapse and multiple organ failure. Extramedullary relapse of ALL in the prostate is very rare. To our knowledge, our case is the first well-documented report in the published work.