Malignant transformation of mature teratomas is rare, occurring in 1%-3% of cases, and no standard treatment has been established. According to the latest World Health Organization (WHO) histological classification, germ cell tumors with a somatic malignant component (sarcoma, carcinoma, or both) are defined as germ cell tumors with somatic malignancy (GCTSM) and have a poor prognosis. Herein, we report a rare case of a mediastinal mature cystic teratoma that showed malignant transformation. The patient achieved complete remission with multimodal treatment. A 74-year-old female non-smoker presented with a main complaint of right-sided chest pain. Twelve years ago, when she visited a local doctor complaining of a cold, an abnormal shadow was detected on chest computed tomography (CT), but she did not seek treatment at that time. The chest radiography performed now revealed increasing right pleural effusion, and the chest CT suggested a mediastinal tumor; therefore, the patient was referred to our department. Chest CT revealed a large mass shadow with calcification in the anterior mediastinum. Although the anterior mediastinal tumor was suspected to have infiltrated the right middle and lower lobes, parietal pleura, and pericardium, complete resection was possible, and surgery was performed for both diagnosis and treatment. Intraoperative findings showed that the tumor had infiltrated the inferior pulmonary vein, resulting in a gross incomplete resection. The pathological diagnosis was mature teratoma with a primary mediastinal somatic malignant tumor. Three courses of etoposide, cisplatin, and bleomycin (BEP) therapy were administered as primary chemotherapy. Three years have passed since the surgery, and the patient has remained recurrence-free. As in this case, BEP therapy is effective in some cases, and further accumulation of such cases will be necessary in the future.