A 58-year-old man who had been undergoing treatment for chronic pericarditis for 15 months was found to have a calcified mediastinal mass shadow on a chest X-ray and was referred to us for further study and treatment. Chest computed tomography and magnetic resonance imaging showed an anterior mediastinal multilocular tumor with rim calcification. Tumor markers were within normal limits. The patient was preoperatively diagnosed as having a calcified mediastinal cyst, and median sternotomy was performed to remove the cystic tumor and the surrounding thymic tissues. Histological examination revealed that the cyst wall consisted of dense fibrous tissue and calcification without epithelial cell lining on the inner surface. Thymic tissue was present around the cyst wall. Although we suspected a thymic cyst from the intraoperative findings, the final diagnosis was made as mediastinal cyst with rim calcification. We herein present a rare case of mediastinal cyst with rim calcification.