We report a case of an undifferentiated pleomorphic sarcoma in a 73-year-old female, with a solitary lung metastasis involving the pericardium that progressed on first-line chemotherapy. Partial removal of the lesion was achieved after lingular segmentectomy, which required en-bloc pericardial resection due to deep pericardial invasion. However, the residual disease significantly grew despite second-line chemotherapy, and the tumor became unresectable due to near encasement of the left anterior descending coronary artery. Therefore, we opted for a salvage radical dose of intensity-modulated radiotherapy (60Gy in 25 fractions) to the pericardial lesion. No acute side effects were observed, and after three years of follow-up, good local control has been achieved with no significant late effects observed. This case suggests that radical radiotherapy using IMRT could be considered to treat sarcomatous pericardial lesions in patients who do not respond to chemotherapy and who are inoperable or non-resectable.