75 year-old man followed up regularly for the treatment of lung cancer came to our hospital with a chief complaint of general malaise. Blood test results showed deterioration in the renal function, and computed tomography (CT) confirmed left hydronephrosis. He was admitted to the hospital with the diagnosis of obstructive pyelonephritis. Despite antibiotic therapy after the left ureteral stent placement, CT on day 19 of hospitalization showed an enlarged soft tissue shadow along the renal pelvis and ureter, which was suspected to be peripelvic urinary extravasation caused by stent occlusion. We decided that conservative treatment would not improve his condition and conducted surgical therapy considering the possibility of malignancy. Intraoperatively, viscous and fragile tumor affected the renal pelvis and ureter. The operation resulted in left nephrectomy because radical resection was impossible. The pathological diagnosis was sarcomatoid urothelial carcinoma of the renal pelvis with ureter origin. He died due to multipleorgan failureon day 20 after theope ration. Were port a caseof sarcomatoid urothelial carcinoma in the upper urinary tract that was difficult to diagnose preoperatively based on imaging studies.