Inclusion Criteria  Has histologically confirmed diagnosis of advanced (metastatic) and/or unresectable (locally advanced) biliary tract cancer (intra-or extrahepatic cholangiocarcinoma or gallbladder cancer) Has measurable disease based on Response Evaluation Criteria in Solid Tumors (RECIST 1.1), as determined by the site investigator Participants with a history of hepatitis B or hepatitis C can be enrolled if they meet study criteria Is able to provide archival tumor tissue sample or newly obtained core or excisional biopsy of a tumor lesion Has a life expectancy of greater than 3 months Has adequate organ function  Exclusion Criteria  Has had previous systemic therapy for advanced (metastatic) or unresectable (locally advanced) biliary tract cancer (intra-or extra hepatic cholangiocarcinoma or gallbladder cancer) Has ampullary cancer Has small cell cancer, neuroendocrine tumors, lymphoma, sarcoma, mixed tumor histology and/or mucinous cystic neoplasms Has received prior therapy with an anti-programmed cell death 1 (anti-PD-1), anti- programmed cell death ligand 1 or 2 (anti-PD-L1, anti-PD-L2) agent or with an agent directed to another stimulatory or coinhibitory T-cell receptor (e.g., cytotoxic T-lymphocyte-associated protein 4 [CTLA-4], OX-40, CD137) Has a known history of, or any evidence of, central nervous system (CNS) metastases and/or carcinomatous meningitis, as assessed by local site investigator Has had an allogenic tissue/solid organ transplant