Abstract: A 28-year-old female patient was referred to and admitted in our hospital for presence of anterior mediastinal mass for 4 years. Enchanced chest computed tomography (CT) revealed an anterior mediastinal mass of soft-tissue density measuring 7.1 cm×3.8 cm with slight homogeneous enhancement after intravenous administration of contrast agent. The mass was clinically considered a thymoma. Then, surgical excision of anterior mediastinal mass was performed under general anesthesia. Postoperative histopathology revealed that there were foamy histiocyte clusters on the background of fibrous tissue hyperplasia and hyaline, with lymphoid hyperplasia, infiltration of plasma cells, and the presence of emperipolesis of lymphocytes and plasma cells in the tissue cells. Immunohistochemistry showed S100 protein (+), cluster of differentiation (CD) 68 (+), CD163 (+), immunoglobulin G4 (+), and CD1a (−). Eventually, confirmed diagnosis of extranodal Rosai-Dorfman disease was made. The patient showed no clinical symptoms and no recurrence was found on CT images over the 3-year followup. In clinical practice, this disease should be differentiated from other anterior mediastinal masses such as thymoma, lymphoma, and teratoma.