We report a case of a seminal vesicle cyst supposed to be associated with prostate cancer in a 79-year-old Japanese man presenting with urinary retention. A fist-sized soft mass was palpated at the anterior wall of the rectum and serum prostate-specific antigen (PSA) was elevated to 59.8 ng/ml. Transrectal ultrasonography, computed tomography and magnetic resonance imaging revealed a retrovesical cystic mass measuring 7 cm in diameter and the absence of bilateral seminal vesicles. On vasography the lumen of the cystic lesion was visualized immediately, but the radiopaque fluid did not flow into the urethra. Transperineal prostate biopsy revealed moderately differentiated adenocarcinoma and puncture of the cyst revealed bloody fluid including sperm with a low PSA level. These findings strongly suggested that the mass was a seminal vesicle cyst caused by ejaculatory duct obstruction associated with prostate cancer. He has received endocrine therapy with goserelin acetate and bicalutamide for 6 months with no enlargement of the cystic lesion.