We report a very rare case of an ectopic thyroid in the superior mediastinum, which was detected incidentally using imaging. The case was a 50-year-old woman patient. She had an orthotopic thyroid and normal thyroid function. This superior mediastinum mass obviously lacked continuity with the orthotopic thyroid. Its computed tomography density was lower than that of the orthotopic thyroid, and an enhancement was heterogeneously observed. In the cytodiagnosis, only large and small lymphocytes were observed, and malignant diseases such as malignant lymphoma could not be ruled out, so surgical resection was performed through a cervical incision. Combined resection of the thyroid was unnecessary, and ligation of the feeding vessels from the thorax side was able to be carried out without incident.