This is a multicenter, randomized, double-blind, placebo-controlled study of niraparib plus pembrolizumab versus placebo plus pembrolizumab as maintenance therapy in participants with advanced or metastatic non-small cell lung cancer (NSCLC) who have achieved stable disease (SD), partial response (PR), or complete response (CR) following completion of standard of care first-line platinum-based induction chemotherapy with pembrolizumab. The primary hypotheses are: participants with confirmed diagnosis of NSCLC could benefit from niraparib plus pembrolizumab versus placebo plus pembrolizumab with respect to Progression-free survival (PFS) and Overall survival (OS).