Inflammatory myofibroblastic tumor (IMT) is a rare mesenchymal neoplasm of intermediate biologic potential, most commonly arising in the lung but capable of occurring at extrapulmonary sites. Primary osseous involvement is rare and may radiographically simulate high-grade sarcoma. We report the case of a 19-year-old male presenting with progressive left knee pain and swelling. Imaging demonstrated an aggressive proximal tibial lesion with cortical destruction, periosteal reaction, suspected osteoid-type matrix mineralization, and soft-tissue extension. Systemic staging revealed bilateral pulmonary nodules and thoracic and lumbar vertebral lesions, highly suspicious for metastatic osteosarcoma. Core needle biopsy with immunohistochemistry and fluorescence in situ hybridization confirmed ALK-positive inflammatory myofibroblastic tumor. During follow-up, the patient developed a secondary pathologic avulsion fracture of the tibial tuberosity due to tumor-related structural weakening. This case highlights the importance of histopathologic and molecular confirmation prior to definitive oncologic therapy in aggressive-appearing bone tumors.