Tislelizumab, an anti-programmed death protein-1 (PD-1) monoclonal antibody, was engineered to minimize binding to FcγR on macrophages to abrogate antibody-dependent phagocytosis, a mechanism of T-cell clearance and potential resistance to anti-PD-1 therapy. This single-arm phase 2 trial (NCT04004221/CTR20170071) assessed the safety, tolerability, and efficacy of tislelizumab in patients with PD-L1 positive urothelial carcinoma who progressed during/following platinum-containing therapy and had no prior PD-(L)1 inhibitor treatment. Patients were considered PD-L1-positive if ≥25% of tumor/immune cells expressed PD-L1 using the VENTANA™ PD-L1 (SP263) assay. The primary endpoint was objective response rate by independent review committee. As of September 16, 2019, 113 patients had a median study follow-up time of 9.4 months. Most patients (76%) had visceral metastases, including 24% with liver and 23% with bone metastases. Among 104 efficacy-evaluable patients, confirmed objective response rate was 24% (95% CI, 16, 33), including 10 complete and 15 partial responses. Median duration of response was not reached. Among 25 responders, 17/25 (68%) had ongoing responses. Median progression-free survival and overall survival were 2.1 and 9.8 months, respectively. The most common treatment-related adverse events were anemia (27%) and pyrexia (19%). Anemia (7%) and hyponatremia (5%) were the only grade 3-4 treatment-related adverse events occurring in ≥5% of patients. Three investigator-assessed deaths were considered possibly related to study treatment (hepatic failure, n = 2; respiratory arrest, n = 1). Tislelizumab demonstrated meaningful clinical benefits in patients with previously treated locally advanced or metastatic PD-L1-positive urothelial carcinoma and had a manageable safety profile. This article is protected by copyright. All rights reserved.