Background: Concurrent chemoradiotherapy (CRT) is a recommended treatment option for patients (pts) with MIBC who seek a bladder-preserving alternative to radical cystectomy and/or not candidates for radical cystectomy. The anti–PD-1 antibody pembrolizumab is approved for metastatic bladder cancer and non–muscle-invasive bladder cancer (NMIBC). The randomized, double-blind, phase 3 KEYNOTE-992 study (NCT04241185) is evaluating efficacy and safety of pembrolizumab plus CRT versus placebo plus CRT in pts with MIBC who chose bladder preservation. Methods: Eligibility criteria include adultswho have a histologically confirmed initial diagnosis of MIBC (T2-T4aN0M0) with ≥50% urothelial histology, ECOG PS score of 0 to 2, have received no prior therapy with anti–PD-1/L1 or agents directed to another stimulatory or coinhibitory T-cell receptor, and have planned treatment with one of the specified radiosensitizing chemotherapies. Prior treatment for NMIBC with intravesical instillation therapy such as BCG or intravesical chemotherapy is permitted. Approximately 636 pts will be randomly assigned 1:1 to receive pembrolizumab 400 mg IV or placebo IV every 6 weeks for up to 9 cycles, both with CRT. Investigator’s choice of chemotherapy will be specified before random assignment. Pts will be stratified by ECOG PS (0 and 1 vs 2), PD-L1 combined positive score (<10 vs ≥10), T stage (T2 vs T3 and T4), and geographic region (United States vs European Union vs rest of the world). Radiosensitizing chemotherapy will be cisplatin monotherapy (35 mg/m² IV weekly on day 1), 5-fluorouracil (500 mg/m² on days 1-5 and days 22-26) plus mitomycin C (12 mg/m² on day 1), or gemcitabine monotherapy (27 mg/m² IV twice weekly on days 1 and 4). Radiation therapy will be conventional radiotherapy consisting of 64 Gy at 2 Gy/fraction over 6.5 weeks (whole bladder with or without pelvic nodes) or hypofractionated radiotherapy consisting of 55 Gy at 2.75 Gy/fraction over 4 weeks (whole bladder only). Imaging via CT and CT urography or magnetic resonance urography will be performed 10 weeks after completion of CRT, every 12 weeks until the end of year 2, then every 24 weeks thereafter. Adverse events (AEs) will be monitored throughout the study and up to 30 days after cessation of treatment (90 days for serious AEs). Primary end point is bladder-intact event-free survival (defined as time from random assignment to first documented occurrence of residual/recurrent MIBC, nodal or distant metastases, radical cystectomy, or death from any cause) as assessed by blinded independent central review and/or central pathology review. Secondary end points are overall survival, metastasis-free survival, time to cystectomy, time to occurrence of NMIBC, safety and tolerability, and patient-reported outcomes. Enrollment is ongoing in Asia, Australia, Europe, North America, and South America. Clinical trial information: NCT04241185.