Bacillus Calmette–Guérin (BCG) induction and maintenance (I+M) after transurethral resection of bladder tumor is standard of care (SOC) in high-risk non-muscle invasive bladder cancer (NMIBC). However, disease recurrence/progression occurs in approximately 40% of patients at 2 years, with unfavorable prognosis. Limited bladder-sparing therapeutic options exist, and no improvements to response durability have been observed in decades. CREST is a global, phase 3, randomized trial evaluating subcutaneous sasanlimab in combination with BCG-I+M (Arm A), sasanlimab in combination with BCG-I (Arm B) or BCG-I+M (Arm C) in BCG-naive high-risk NMIBC. The primary endpoint was investigator-assessed event-free survival (EFS) for Arm A versus Arm C; key secondary endpoints were EFS (Arm B versus Arm C) and overall survival. Patients were randomized 1:1:1 to Arm A (N = 352), Arm B (N = 352) and Arm C (N = 351). The trial met its primary endpoint with a statistically significant and clinically meaningful prolongation of EFS (Arm A versus Arm C); hazard ratio, 0.68 (95% confidence interval: 0.49–0.94); one-sided P = 0.0095. The 36-month estimated EFS rates were 82.1% (Arm A) and 74.8% (Arm C). EFS benefit for Arm A versus Arm C was observed across prespecified subgroups, including carcinoma in situ (CIS) and T1. The safety profile of the combination was consistent with the known profiles. To our knowledge, sasanlimab is the first anti-PD-1 antibody to show a clinically meaningful prolongation of EFS when combined with BCG-I+M versus SOC in patients with BCG-naive high-risk NMIBC. Sasanlimab combined with BCG-I+M has the potential to redefine the treatment paradigm and clinical decision-making for patients with BCG-naive high-risk NMIBC. ClinicalTrials.gov identifier: NCT04165317.