A 65-year-old female patient was admitted to our hospital presenting with a superior mediastinal big mass that was elastic, hard, and painless. Laboratory data including serum calcium level and thyroid and parathyroid hormonal functions revealed no abnormalities. Further examination consisting of computed tomography, magnetic resonance imaging, and ultrasonography demonstrated that it was a solid tumor extending into the superior mediastinum. Technetium (Tc-99) sestamibi scan revealed a hypofunctioning focus in that area. The preoperative diagnosis was a thyroid tumor or a metastatic lymph node. Parathyroid carcinoma was suspected on intraoperative frozen pathological examination. The tumor was successfully removed with left thyroid lobectomy, and neck node dissection was performed. Macroscopically, it appeared as a dark reddish solid tumor, and the cut surface presented opalescence. Immunohistology confirmed that there was proliferation of tumor cells with positive chromogranin A staining. Thus, the tumor was diagnosed as parathyroid carcinoma histopathologically despite a lack of clinical evidence for hyperparathyroidism. This patient has been followed with no evidence of recurrence, a normal serum calcium 4 years after surgery, and postoperative radiotherapy. This report describes a case of nonfunctional parathyroid carcinoma with a massive mass that technetium (Tc-99) sestamibi scan failed to detect, and we showed negative immunostaining for parathyroid hormone (PTH) (N).