Undifferentiated pleomorphic sarcoma (UPS) in the mandible region is a rare malignant neoplasm. We encountered the case of a patient who had been diagnosed with deep benign fibrous histiocytoma (DBFH) prior to surgery, but the final biopsy revealed the presence of tumor tissues in the resected margin, which indicated a UPS. Therefore, radiotherapy was conducted post-operatively, but the tumor recurred approximately 3 years later and a second surgery was performed. The difference between the initial and final diagnoses was considered to be due to the characteristics of the tumor, as it involved both benign and malignant components. DBFH and UPS can occur in young women. DBFH can transform to a malignancy and UPS may also appear as a benign tumor during early biopsy. Hence, attention should be paid to diagnosis and treatment methods, and ongoing close observation is required.