True primary mediastinal ectopic goiters are quite rare, and they occur in less than 1% of all goiters. We report here on a case in which a primary mediastinal ectopic goiter was accompanied with papillary microcarcinoma of the cervical thyroid. The mediastinal ectopic goiter was not connected to the cervical thyroid and its blood was supplied from intrathoracic vessels, which necessitated a thoracotomy or sternotomy for removal rather than a standard cervical collar incision. We performed total thyroidectomy for the papillary microcarcinoma of the cervical thyroid gland and transsternal removal for the mediastinal goiter; this resulted in a favorable outcome.