Duplication of the bladder is an unusual congenital anomaly. Complete and incomplete forms are well defined in published reports. We present an unusual case of a 3-year-old boy with two separate-walled bladders, one lying in front of the other, communicating through a tiny septum, at the neck of the primary bladder that drained to a normal single urethra. This type of isolated bladder anomaly was not defined in the classification of the duplication of the bladder. In patients with a lower abdominal cystic mass, complete bladder duplication must be included in the differential diagnosis of pelvic cystic masses.