An abnormal opacity was found incidentally on chest radiography in a 19-year-old Japanese man with lumbago. Chest computed tomography (CT) revealed a 55 × 40 mm well-defined mass with smooth margins in the lower lobe of the right lung (S6). Contrast-enhanced dynamic CT showed that the lesion was not enhanced, suggesting a cystic lesion, such as a bronchogenic cyst. However, contrast-enhanced dynamic magnetic resonance imaging (MRI) revealed enhancement of the lesion, which had a clear margin and homogenous consistency, suggesting a benign pulmonary solid tumor. Video-assisted thoracoscopic surgery, S6 segmentectomy, was performed, disclosing the lesion to be solid. Histopathologically, the tumor consists of spindle cells with ample eosinophilic cytoplasm arranged in an interlacing pattern. Immunohistochemical staining revealed positive desmin reactivity but was negative for α smooth muscle actin and HHF-35 (muscle specific actin), indicating atypical phenotypic differentiation. The lesion was accordingly diagnosed as leiomyosarcoma with low grade malignancy, and right lower lobectomy was performed. The patient has remained free from recurrence for 18 months without any additional chemotherapy or radiation therapy. Contrast-enhanced dynamic MRI should be carried out to facilitate accurate preoperative diagnosis and selection of the correct surgical plan.