Primary thyroid leiomyosarcoma (LMS) is a rare tumor type with an unusual location, the diagnosis is based entirely on histological and immunohistochemical evaluations. In the present study, a rare case of a 74-year-old female patient who exhibited a right anterior neck mass for 12 months, which rapidly enlarged for the last 3 months. Ultrasound of the thyroid revealed a 55×42 mm hypoechoic mass with clear margins in the right lobe. Histological examination of the tumor demonstrated malignant spindle cells in interlacing fascicles and whorls. Additionally, nuclear pleomorphism, tumor giant cells, necrosis and abnormal mitotic figures were observed. The immunohistochemistry indicated that the tumor cells were strongly positive for smooth muscle actin, desmin, p53 and vimentin expression, but negative for cytokeratin, epithelial membrane antigen, thyroid transcription factor-1, paired box-8, 34βE12, cytokeratin 5/6, cluster of differentiation (CD)117, myoglobin, S100, p16. The final histopathological diagnosis was primary thyroid LMS.