The authors present a case report of a giant Müllerian duct cyst in the perineum. A 37-year-old man presented with a mass with the size of 50 cm × 40 cm × 30 cm in the perineum. Computed tomography (CT) scan and magnetic resonance imaging (MRI) of the pelvis and perineum found a huge multilocular cystic mass. X-ray film of the pelvis showed a giant soft-tissue shadow of high density in the perineum. Voiding cystourethrogram did not show any cyst interlinked to posterior urethra. After related examinations, the patient underwent open surgical resection of the cyst. Pathologic examination confirmed the presence of a Müllerian duct cyst. Huge Müllerian duct cyst is uncommon in clinic. Treatments of Müllerian duct cysts depend on their sizes and symptoms.