Osteosarcoma is the most common bone sarcoma in adolescents and children. Tumor thrombus arising from osteosarcoma is rare. We describe the case of a 13-year-old girl with osteosarcoma of the right femur, in whom the 18F-NaF PET/CT was done for initial staging to look for skeletal metastases. The scan showed abnormal increased tracer uptake in the primary tumor and the right common femoral and external iliac vein representing a tumor thrombus. Our case emphasizes the importance of extraosseous findings on 18F-NaF PET/CT, which may result in important management changes.