The anomalous retro-psoas iliac artery is a highly uncommon congenital anomaly amongst the iliolumbar vascular variations. We incidentally encountered a case in which was observed a right retro-psoas iliac artery located between the right psoas muscles and adjacent vertebral bodies. The subject was a 46-year-old female patient diagnosed with urothelial carcinoma of the urinary bladder and left ureter on computed tomography (CT). The patient had experienced no clinical symptom or sign in the lower extremities. Although the retro-psoas iliac artery is an extremely rare iliolumbar vascular anomaly, this diagnosis should be kept in mind in order to avoid severe vascular complications in the event of invasive procedures.