Primary thyroid osteosarcoma is an extremely rare tumor which is associated with a poor prognosis. In this study, we describe an additional case. A 4.5 × 3.8 cm irregular heterogeneous nodule was examined in the left thyroid gland of a 72-year-old woman. Cytological smears and histopathological specimens showed typical features of osteosarcoma with a neoplastic lesion rich in spindle cells with occasional multinucleated cells and lace-like osteoid matrix. Negative immunoreaction with epithelial markers and positive immunoreaction with SATB2 and low Ki-67 labeling index suggested the diagnosis of osteosarcoma. Multiple KMT2C gene mutations determined by next-generation sequencing further confirmed the diagnosis.