Objective: Primary undifferentiated pleomorphic sarcoma (UPS) of the bone is rare. However, the common sites are the knee and proximal femur and humerus, while spinal involvement is rare. We report a case of primary UPS of the 11th thoracic vertebra, where corpectomy would have been difficult and extensive, treated with carbon ion radiotherapy.Case report: A 76-year-old man presented with an osteolytic tumor of the 11th thoracic vertebra on plain computed tomography (CT). The spinal cord was compressed and displaced posteriorly by the tumor on magnetic resonance imaging (MRI), and extraosseous extension was observed. An incisional biopsy was performed, and primary UPS of the 11th thoracic vertebra was diagnosed pathologically. Total en bloc spondylectomy was considered to be challenging because of the extraosseous extension and the patient’s age; thus, carbon ion radiotherapy (70.4 GyE / 32 fraction) was performed. Denosumab (120 mg) was administered subcutaneously every four weeks. No adjuvant chemotherapy was administered. Four years post-treatment, imaging revealed a compression fracture of the 11th thoracic vertebra, but there was no recurrence.Conclusion: Despite a poor prognosis and an aggressive course of UPS of the spine, the tumor continues to be controlled without local recurrence four years after carbon ion radiotherapy.