We report a case of primary malignant lymphoma of the prostate. An 84-year-old man was referred to our hospital with a chief complaint of urinary retention. Magnetic resonance imaging showed a large mass below the bladder and in front of the rectum. Histological and immunocytochemical studies of transperineal biopsy of the prostate showed diffuse large B-cell non-Hogkin's lymphoma. Radiological assessment of the disease confirmed stage IV according to the Ann Arbor classification. Although the tumor was markedly reduced in size after four cycles of combination chemotherapy with cyclophosphamide, adriamycin, vincristine, and prednisolone, he died with brain metastasis 4 months after the diagnosis.