Glomus tumors are relatively uncommon clinically benign tumors. Malignant glomus tumors are rare, and only a small number develop metastatic foci. The usual location is deep dermis or subcutis, but it has been reported in various locations. A 55-year-old man presented with an incidentally found thyroid mass. Neck ultrasound showed a mass with a heterogeneous hypoechoic calcific mass in the right lobe. Right lobectomy specimen showed the 3.6-cm-sized calcified mass composed of sheets of uniform round to polygonal cells and intervening staghorn-shaped vessels. Occasional cellular atypism and necrosis with increased mitotic activity (up to 7 per 10 high-power fields) were found. Infiltration to the residual thyroid parenchyma, vessel, thyroidal capsule, and strap muscle was found. These tumor cells were strongly positive for smooth muscle actin, collagen type IV, and vimentin with pericellular reticulin-cuffing. Ultrastructurally, closely packed oval-shaped tumor cells having cytoplasmic mitochondria, rough endoplasmic reticulums with pinocytotic vesicles along the plasmalemmal surface and thin filaments of 6 nm in diameter were surrounded by thick basal lamina. That mass was diagnosed as a malignant glomus tumor. Incidentally, a 0.5-cm-sized papillary carcinoma was found through entire embedding. Complete thyroidectomy with chemotherapy was done. Thirty months later, multiple metastases developed in the brain and lung, and he expired. To our knowledge, neither benign nor malignant thyroid glomus tumor has been previously described. Here, we describe the first case of a malignant glomus tumor in the thyroid gland.