We report a case of thymic carcinoma with massive pericardial effusion in a 74-year-old man. This patient with dyspnea was referred to our hospital because of pericardial effusion detected by echocardiography. A chest computed tomography and magnetic resonance imaging showed an anterior mediastinal mass measuring 8.0 x 5.0 cm with massive pericardial effusion. The mass lesion was suggestive of thymic carcinoma or invasive thymoma. Initially, he underwent pericardial drainage. The collected fluid was serous and yellow, and cytological examination found no malignant cells. The tumor with partial pericardium was resected. Histopathological findings confirmed the lesion to be squamous cell carcinoma of the thymus. The etiology of a massive nonmalignant pericardial effusion associated with thymic carcinoma warrants further studies. The patient is alive without recurrence and without pericardial effusion at 3 years to date after the operation. Not all pericardial effusion associated with thymic cancer involves malignant effusion.