Phosphaturic mesenchymal tumor, mixed connective tissue variant (PMT-MCT) causes tumor-induced osteomalacia (TIO). Most cases follow a benign clinical course, with rare occurrences of malignant transformation. We report a case of malignant PMT-MCT and review previous malignant cases to identify predictive factors for transformation. A 13-yr-old female, who presented with hypophosphatemic rickets, elevated serum intact fibroblast growth factor 23 (FGF23) levels, and a nodule in the back, received a diagnosis of TIO because of the benign PMT histopathology. After resection of the primary tumor, regular imaging analyses did not indicate any relapse. At 17 years of age, a tumor developed in the left leg and increased in size. The resected tumor showed a histopathology of pleomorphic sarcoma positive for the TP53 mutation. Despite amputation of the affected leg, the patient died due to multiple metastases at 18 years of age. A literature review revealed that 14 out of 15 reported malignant PMT-MCT tumors occurred in adults, and found no predictive factors for malignant transformation and treatment outcome. Changes in size or number of the tumors along with intact FGF23 levels have been considered as the only sign of malignant transformation. This pediatric case report and literature review indicate the need for prolonged regular monitoring for PMT-MCT.