Inclusion Criteria - Participants must have a histologically- or cytologically-confirmed diagnoses of non-small cell lung cancer (NSCLC) with unresectable Stage III disease. - Participants must have received at least 2 cycles of platinum-based concurrent chemotherapy (a total dose of radiation of at least 54 Gy). - Participants must have no progressive disease (PD) following treatment with concurrent chemoradiotherapy (CCRT). - Participants must have an Eastern Cooperative Oncology Group (ECOG) performance status (PS) score of 0 or 1. Exclusion Criteria - Participants with non-squamous histology must not have documented Epidermal Growth Factor Receptor (EGFR) and anaplastic lymphoma kinase (ALK) rearrangements. - Participants must not have an active autoimmune disease. - Participants must not have significant cardiovascular impairment such as uncontrolled hypertension (despite optimal medical treatment), congestive heart failure, active coronary disease (within 6 months prior to randomization), ventricular arrhythmias, major thrombotic or embolic events or major hemorrhagic events within 6 months prior to randomization, or significant risk of pulmonary hemorrhage. - Participants must not have advanced/clinically significant lung disease (within 6 months prior to randomization) or history of interstitial lung disease (ILD) or pneumonitis requiring treatment with systemic steroids (≥ Grade 2), or current or suspected ILD or pneumonitis. - Participants must not have any prior anticancer therapy (outside of CCRT) for locally advanced Stage III disease. - Other protocol-defined Inclusion/Exclusion criteria apply.