We report a rare case of mediastinal mature teratoma with rupture due to blunt trauma. A 15-year-old boy had received a strong head-butt in the left upper chest wall and was admitted with the sudden onset of left-sided severe chest pain and dyspnea. Chest computed tomography (CT) scan on admission revealed a heterogeneous mass lesion in the anterior mediastinum. The contrast-enhanced CT scans obtained 2 days after admission showed not only mediastinal mass lesion but also left pleural effusion. On the radiologic finding at 5 months later, the lesion became larger and was thought to be a typical mediastinal mature teratoma. The patient underwent extirpation of the tumor. Microscopically, the tumor was typically composed of adult-type tissues and was confirmed to be mature teratoma.