A 14 mo old pug was referred because of a large space occupying mass within the thoracic cavity. The patient was worked up with thoracic radiographs, cytology, and computed tomography. The most likely differential diagnosis was thymoma; therefore, a median sternotomy was performed, and the mass was bluntly dissected from the adjacent tissues. The patient recovered uneventfully from surgery and was discharged the next day. Histopathology diagnosed a thymoma with central cystic lesions. Thymomas usually occur in elderly patients and these patients are usually presented because of paraneoplastic signs. The age and signs at presentation, as well as the histopathologic appearance of the presented patient is quite atypical. According to telephone follow-ups, the patient was doing well 9 mo postoperatively.