Primary thymic mucinous adenocarcinoma is extremely rare; to our knowledge, only 16 cases have been reported to date. A 68-year-old man presented to a previous hospital due to massive pericardial effusion. Cytological examination of the pericardial effusion revealed the presence of adenocarcinoma, and computed tomography showed an anterior mediastinal mass lesion invading the pericardium. Because systemic examination failed to detect other lesions, except for liver metastasis, mediastinal lymph node swelling, and pleural dissemination, a thoracoscopic biopsy of the mediastinal and pleural tumor was performed. The pathological diagnosis was thymic mucinous adenocarcinoma. Although he received chemotherapy, he died due to cancer 6 months after the biopsy.