This prespecified subanalysis of the global, randomized controlled phase III KEYNOTE-024 study of pembrolizumab vs chemotherapy in previously untreated metastatic non-small-cell lung cancer without EGFR/ALK alterations and a programmed death ligand 1 (PD-L1) tumor proportion score of 50% or higher evaluated clinical outcomes among patients enrolled in Japan. Treatment consisted of pembrolizumab 200 mg every 3 weeks (35 cycles) or platinum-based chemotherapy (four to six cycles). The primary end-point was progression-free survival; secondary end-points included overall survival and safety. Of 305 patients randomized in KEYNOTE-024 overall, 40 patients were enrolled in Japan (all received treatment: pembrolizumab, n = 21; chemotherapy, n = 19). Median progression-free survival was 41.4 (95% confidence interval [CI], 4.2-42.5) months with pembrolizumab and 4.1 (95% CI, 2.8-8.3) months with chemotherapy (hazard ratio [HR], 0.27 [95% CI, 0.11-0.65]; one-sided, nominal P = .001). Median overall survival was not reached (NR) (95% CI, 22.9-NR) and 21.5 (95% CI, 5.2-35.0) months, respectively (HR, 0.39 [95% CI, 0.17-0.91]; one-sided, nominal P = .012). Treatment-related adverse events occurred in 21/21 (100%) pembrolizumab-treated and 18/19 (95%) chemotherapy-treated patients; eight patients (38%) and nine patients (47%), respectively, had grade 3-5 events. Immune-mediated adverse events and infusion reactions occurred in 11 pembrolizumab-treated patients (52%) and four chemotherapy-treated patients (21%), respectively; four patients (19%) and one patient (5%), respectively, had grade 3-5 events. Consistent with results from KEYNOTE-024 overall, first-line pembrolizumab improved progression-free survival and overall survival vs chemotherapy with manageable safety among Japanese patients with metastatic non-small-cell lung cancer without EGFR/ALK alterations and a PD-L1 tumor proportion score of 50% or higher. The trial is registered with Clinicaltrials.gov: NCT02142738.