Osteochondroma is a common bone tumor but a rare tumor in the rib. It is often asymptomatic and observed incidentally. This is a case report of a 49-year-old woman with an osteochondroma mimicking a mediastinal mass in hereditary multiple exostoses. The chest X-ray and computed tomography (CT) scans revealed the bony density feature of the mass. Surgical excision confirmed that the lesion was an osteochondroma.