Phosphaturic mesenchymal tumor is a rare neoplasm that produces fibroblast growth factor 23 resulting in the paraneoplastic tumor-induced osteomalacia. Patients are typically diagnosed several years after the first signs of bone pain, fatigue, and weakness; the non-specific character of the symptoms often delays the diagnosis and curative surgical treatment. The following case report details a classic presentation of a phosphaturic mesenchymal tumor of mixed connective tissue type involving the right posterior tibialis muscle with erosion of the adjacent medial fibular cortex. The patient underwent a surgical resection and is currently asymptomatic. This report highlights the need for multidisciplinary diagnostic effort, effectiveness, of early surgical intervention, and emphasizes the importance of including this rare but damaging disease process in the differential diagnosis of osteomalacia.