A 70-year-old Japanese man with stage IV EGFR-mutated lung adenocarcinoma complained of right mild back pain. The patient had been heavily treated with several cytotoxic or molecular targeted agents for 10 years and received a palliative radiation therapy of 2nd sacral vertebra 5 years ago. Computed tomography showed the abnormal lesion in right iliopsoas muscle. A pathological examination confirmed undifferentiated pleomorphic sarcoma, consistent with the diagnosis of radiation-induced sarcoma (RIS). Since RIS is a rare late-onset complication of radiation therapy, to our knowledge, this is the first report of RIS that was associated with advanced lung cancer and detected after palliative radiation therapy. The careful long-term follow-up is thus necessary even after palliative radiation therapy and we have to be aware of the existence of RIS.