Tumor-induced osteomalacia is a rare clinical entity that is associated with soft-tissue or skeletal tumors. We present a case report of a patient with a chest wall mesenchymal chondrosarcoma who presented with bone pain. The patient had skeletal changes in the femoral neck and fibula consistent with osteomalacia and laboratory values suggesting phosphate diabetes. The patient was treated with tumor resection and phosphate supplementation with reversal of the signs and symptoms of osteomalacia. Tumor-induced osteomalacia is vitamin-D-resistant and often reversed by complete removal of the tumor. Most commonly, the causative tumors are of vascular, mesenchymal, or fibrous origin. The osteomalacia is associated with bone pain, muscle weakness, and radiographic changes. Tumor-induced humoral factors have been implicated in causing the osteomalacia, but the definite etiology has yet to be determined. Current treatment includes complete tumor resection and electrolyte supplementation.