Coexistence of a mucinous cystadenoma arising in a mature cystic teratoma is infrequently reported. Herein a case of a 25-year-old woman diagnosed with a right ovarian mucinous cystadenoma arising in a mature cystic teratoma is reported. She presented with lower right abdominal discomfort. Ultrasound showed a 14.8 × 7.9 × 12.5 cm structure on the right adnexa. She underwent a diagnostic laparoscopy, which was converted to exploratory laparotomy, during which a right salpingo-oophorectomy was performed.