Thymic immunoglobulin G4 (IgG4) is a rare entity that can mimic anterior mediastinal tumors. Preoperative diagnosis is essential to prevent unnecessary surgery. Little is known about disease presentation, diagnosis, and response to therapy. A 40-year-old man presented with retrosternal chest pain. A roentgenogram showed an anterior mediastinal mass. Intraoperatively, dense adhesions to the phrenic nerve, aorta, and pulmonary artery deemed the tumor unresectable. Thymic IgG4 disease was diagnosed on the basis of tissue biopsy. The patient's postoperative serum IgG4 level was elevated, and no other organ was affected. The patient responded to prednisolone therapy, and after 18 months of follow-up, there was no recurrence or involvement of other organs.