This is a prospective, single-center, single-arm phase II study evaluating whether a single postoperative intravesical instillation of disitamab vedotin (RC48) can reduce bladder recurrence after radical nephroureterectomy (RNU) in patients with HER2-positive upper tract urothelial carcinoma (UTUC). Eligible patients will receive one intravesical instillation of RC48 within 96 hours after surgery. The primary objective is to determine the 12-month intravesical recurrence rate. Serial urine samples will be prospectively collected to evaluate urinary methylation biomarkers for early recurrence detection.