A case of vesical endometriosis associated with unilateral renal agenesis and uterus didelphys (Wunderlich syndrome) is reported. A 20-year-old unmarried woman visited the gynecologic department with dysmenorrhea. She was referred to us because a bladder mass was detected on abdominal ultrasonography. She had no episode of gross hematuria. Cystoscopy revealed a finger tip-sized cystic mass dark red to dark blue in color. Computerized tomography and magnetic resonance imaging demonstrated right renal agenesis and uterus didelphys. The mass was located at the ureterovesical pouch and protruded into the bladder. Partial cystectomy, right hysterectomy and right salpingo-oophorectomy were performed. Histopathological diagnosis was endometriosis. The patient was treated with 400 mg danazol for 6 months has had no recurrence and regular menstruation.