Primary thymic adenocarcinoma is an extremely rare tumor. This report describes a case of thymic adenocarcinoma with a peculiar pathological feature and clinical course. A 54-year-old man complained of cough, dyspnea, and chest pain. Computed tomography scans revealed an anterior mediastinal mass with intracaval growth into the superior vena cava. No definitive diagnosis could be made after several examinations and palliative radiotherapy was performed. He passed away 56 days after the appearance of the initial symptoms. An autopsy revealed that the mediastinal tumor obstructed not only the superior vena cava but also the pulmonary arteries. The histological findings revealed the tumor to be an adenocarcinoma with sarcomatous features. The tumor was immunohistochemically positive for CD5 and negative for thyroid transcription factor-1. These results suggested the tumor to be a thymic adenocarcinoma with sarcomatoid features; furthermore, the tumor invaded the great vessels, thus resulting in a poor prognosis.