Concurrent chemoradiotherapy is a standard therapy for patients with stage III non-small-cell lung cancer (NSCLC). Durvalumab is an approved treatment option following concurrent chemoradiotherapy in the absence of disease progression. The multicenter, phase III, randomized, placebo- and active-controlled, double-blind KEYLYNK-012 study evaluates whether initiation of immunotherapy with pembrolizumab concurrently with chemoradiotherapy, followed by post-chemoradiotherapy pembrolizumab with or without olaparib improves outcomes for participants with stage III NSCLC. (ClinicalTrials.gov: NCT04380636) METHODS: Eligible participants are aged ≥18 years with previously untreated, pathologically confirmed, stages IIIA-C, squamous or nonsquamous NSCLC not suitable for surgery with curative intent. Participants will be randomized 1:1:1 to platinum-doublet chemotherapy plus radiotherapy with pembrolizumab (Groups A and B) or concurrent chemoradiotherapy alone (Group C) for 3 cycles. In the absence of disease progression, participants will receive pembrolizumab plus olaparib placebo (Group A), pembrolizumab plus olaparib (Group B), or durvalumab monotherapy (Group C). Dual primary endpoints are progression-free survival per RECIST version 1.1 by independent central review and overall survival.Enrollment began on July 6, 2020, and is ongoing at approximately 190 sites.KEYLYNK-012 will provide important information on the efficacy and safety of pembrolizumab combined with concurrent chemoradiotherapy and subsequent pembrolizumab with or without olaparib in participants with unresectable stage III NSCLC.