Reported herein is a first case of an iatrogenic retroperitoneal hydatid cyst of Morgagi causing an unusual symptom of left lower extremity numbness.  A 60-year-old woman underwent a hysterectomy (TAH) and bilateral salpingo-oophorectomy (BSO) 16 years previously and used hormone therapy for 10 years after the operation.  A palpable cystic mass (6 cm in diameter) was found, demonstrated by pelvic ultrasonography and magnetic resonance imaging.  The cyst embedded in the pelvic retroperitoneal space was completely excised at our district hospital and the symptom disappeared postoperatively.  Using Masson’s trichrome stain and immunohistochemistry antigens study histologically, we confirmed its origin from Müllerian-type epithelium and diagnosed most likely as a hydatid of Morgagni.  Conclusively, the retroperitoneal hydatid cyst of Morgagni in our case is a secondary cause from incidentally leaving behind this small remnant in the retroperitoneal space when ligating the infundibulopelvic ligament in performing TAH & BSO.