Efficacy and Safety of Pembrolizumab (MK-3475) in Combination With Chemoradiotherapy (CRT) Versus CRT Alone in Muscle-invasive Bladder Cancer (MIBC) (MK-3475-992/KEYNOTE-992)

Inclusion Criteria: * Has a histologically confirmed initial diagnosis of muscle-invasive bladder cancer (MIBC) with predominant urothelial histology * Has clinically nonmetastatic bladder cancer (N0M0) * Has planned and is eligible to receive chemoradiotherapy (CRT) and one of the protocol-specified radiosensitizing chemotherapy regimens * Has Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2 * Demonstrates adequate organ function * Male participants are eligible to participate if they agree to the following during the intervention period and for at least 90 days after the last dose of CRT treatment: * Refrain from donating sperm * Be abstinent from heterosexual intercourse as their preferred and usual lifestyle (abstinent on a long term and persistent basis) and agree to remain abstinent; or must agree to use contraception unless confirmed to be azoospermic * A female participant is eligible to participate if she is not pregnant or breastfeeding, and at least one of the following conditions applies: * Is not a woman of childbearing potential (WOCBP) * Is a WOCBP and using a contraceptive method that is highly effective (with a failure rate of \<1% per year), with low user dependency or be abstinent from heterosexual intercourse as their preferred and usual lifestyle (abstinent on a long term and persistent basis), during the intervention period and for at least 180 days the time needed to eliminate each study intervention after the last dose of study intervention; and agrees not to donate eggs (ova, oocytes) to others or freeze/store for her own use for the purpose of reproduction during this period. The length of time required to continue contraception for each study intervention is as follows: MK-3475 - 120 days and CRT - 180 days Exclusion Criteria: * Has the presence of diffuse carcinoma in situ (CIS) (multiple foci of CIS) throughout the bladder * Has the presence of urothelial carcinoma (UC) at any site outside of the urinary bladder in the previous 2 years except for Ta stage/T1 stage/CIS of the upper tract if the participant has undergone a complete nephroureterectomy * Has a known additional malignancy that is progressing or has required active therapy within the past 3 years, except basal cell carcinoma of the skin, squamous cell carcinoma of the skin, superficial bladder cancer or other carcinoma in situ that has undergone potentially curative therapy * Has the presence of bilateral hydronephrosis * Has limited bladder function with frequency of small amounts of urine (\< 30 mL), urinary incontinence, or requires self-catheterization or a permanent indwelling catheter * Has received prior pelvic/local radiation therapy for any reason or any antineoplastic treatment for muscle-invasive bladder cancer (MIBC). Treatment for non-muscle invasive bladder cancer (NMIBC) with intravesical instillation therapy that was completed ≥28 days prior to randomization is allowed. Prior systemic treatment of NMIBC is not permitted. * Received prior therapy with an anti-PD-1 (programmed cell death protein 1), anti-PD-L1 (programmed death-ligand 1), or anti-PD-L2 (programmed cell death 1 ligand 2), or with an agent directed to another stimulatory or coinhibitory T-cell receptor (e.g., CTLA-4 \[cytotoxic T-lymphocyte-associated protein 4\], OX 40, or CD137 \[cluster of differentiation 137\]) * Has received a live vaccine within 30 days before the first dose of study medication * Is currently participating in or has participated in a study of an investigational agent or has used an investigational device within 4 weeks before the first dose of study medication * Has known severe hypersensitivity (≥Grade 3) to the selected chemotherapy regimen, and/or any of their excipients and excipients of pembrolizumab * Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 7 days before the first dose of study medication * Has an active autoimmune disease that has required systemic treatment in the past 2 years (ie, with use of disease modifying agents, corticosteroids or immunosuppressive drugs). Replacement therapy (eg, thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency) is not considered a form of systemic treatment and is allowed * Has a history of non-infectious pneumonitis that required steroids or has current pneumonitis * Has an active infection requiring systemic therapy * Has a known history of human immunodeficiency virus (HIV) infection * Has a known history of hepatitis B or known active hepatitis C virus infection * Has a known history of active tuberculosis (TB; Bacillus tuberculosis) * Has a known psychiatric or substance abuse disorder that would interfere with the participant's ability to cooperate with the requirements of the study * Has had an allogenic tissue/solid organ transplant