Dedifferentiated parosteal osteosarcoma (dd-POS) is defined as high-grade sarcomatous components coexisting with low-grade POS components. With regard to the histological diagnosis of dd-POS, the sampling of a small area of dedifferentiation through the densely mineralized POS can be a problem. In this situation, imaging is important to identify the area with the highest possibility of dedifferentiation. We report a patient in whom dedifferentiation was shown by computed tomography (CT) and magnetic resonance imaging (MRI). CT revealed a radiolucency in a highly mineralized area. T2-Weighted MRI showed a relatively high signal intensity, corresponding to the radiolucency, surrounded by a very low signal intensity area. Furthermore, gadolinium-enhanced T1-weighted MRI showed marked enhancement. Based on these imaging techniques, the condition was diagnosed as most likely to be a dd-POS, although a representative sample was not accessible by incisional biopsy. Neoadjuvant chemotherapy was administered, followed by wide resection and adjuvant chemotherapy. Four years after the surgery, partial lobulectomy was required because of a pulmonary metastasis. Three years after the second surgery, the patient remained well without evidence of metastases. Based on the initial diagnosis and, consequently, the optimal treatment of combined chemotherapy and wide resection, our patient showed a good clinical outcome.