Sclerosing thymoma is an extremely rare entity described mostly in the recent past and, to our knowledge, described in only 14 cases, including one we recently diagnosed at our institution. The pathogenesis and biologic behavior of this relatively unknown entity is still largely uncertain. However, there are several consistent findings among the cases described so far. It is mostly asymptomatic or patients may present with shortness of breath or chest pain due to a large, well-circumscribed mass in the anterior mediastinum. It has a characteristic homogeneously white-tan, solid cut surface without any necrosis, hemorrhage, or cystic change. Microscopically, it is characterized by extensive sclerosis, hyalinization, and calcification. Only scant evidence of residual thymoma may be present to characterize this entity. Complete surgical resection has, so far, been curative. Knowledge of this rare entity in the general practice of surgical pathologists is important when considering the differential diagnosis of anterior mediastinal lesions.