Tailgut cyst is a rare congenital anomaly arising in the retrorectal space. Malignancy arising from thetailgut cyst is very rare. We experienced a case of adenocarcinoma arising from the tailgut cyst. The findings ofthis rare tumor are bony destruction of the sacrum on plain radiograph, a cystic mass on ultrasound, a lowattenuation mass with calcification and enhancement on CT, and a multiseptated cystic mass containing solidcomponent on MRI.