Metastasis of osteosarcoma to the muscles is extremely rare. 18F-sodium fluoride (NaF) is a bone-seeking agent that can also localize extraosseous calcifying lesions. We report the case of a 33-year-old woman with osteosarcoma of the right fibula treated with surgery and chemotherapy 2 years back who was referred for 18F-NaF PET-CT to evaluate possible metastatic lesions. 18F-NaF PET-CT unexpectedly detected 2 foci of soft tissue increased tracer uptake in the right lateral abdominal and left paraspinal muscles, which corresponded to the focal calcification in the muscles. The pathological examination revealed metastatic osteosarcoma to the muscles.