Abstract Introduction: Malignant transformation of mature cystic teratoma (MCT) is extremely rare and has poor prognosis. We report a case of malignant transformation of ovarian MCT into undifferentiated carcinoma, that showed extremely rapid progression after surgery. Patient concerns: A 51-year-old woman presented with complaints of abdominal swelling, pain and fever. Computed tomography (CT) and transvaginal ultrasonography revealed a cystic mass in the pelvis. Diagnosis: Based on the CT findings, ovarian tumor with possible rupture was provisionally diagnosed. However, post-operative histopathology showed malignant transformation of mature ovarian cystic teratoma into undifferentiated carcinoma. Interventions: Total abdominal hysterectomy with bilateral salpingo-oophorectomy and omentectomy was performed. Outcomes: Positron emission tomography with 18-fluorodeoxyglucose (FDG-PET) after surgery showed multiple FDG uptake in liver, paraaortic lymph nodes and mesentery. Adjuvant chemotherapy was started, but the patient died 3 months after the surgery due to disease progression. Autopsy findings confirmed the metastases of undifferentiated carcinoma in liver, mesentery and left lung. Conclusions: Malignant transformation of MCT into undifferentiated carcinoma can be rapid and has poor outcomes. Pre-operative diagnosis is very difficult. Moreover, limited literature makes it difficult to establish the standardized treatment strategy for these patients. This case report shows that progression of this malignancy can be rapid despite surgery and chemotherapy.