AbstractPembrolizumab plus chemotherapy improved progression‐free survival (PFS) and overall survival (OS) compared with placebo plus chemotherapy in patients with previously untreated locally recurrent inoperable or metastatic triple‐negative breast cancer with tumor programmed cell death ligand 1 (PD‐L1) combined positive score (CPS) ≥10 in the global, phase 3, randomized controlled trial KEYNOTE‐355. We report results for patients enrolled in Japan. Patients were randomized 2:1 to pembrolizumab 200 mg or placebo Q3W for 35 cycles plus chemotherapy (nab‐paclitaxel, paclitaxel, or gemcitabine–carboplatin). Primary endpoints were PFS per RECIST version 1.1 by blinded independent central review and OS in patients with PD‐L1 CPS ≥10, PD‐L1 CPS ≥1, and the intention‐to‐treat (ITT) population. No alpha was assigned to this exploratory analysis. Eighty‐seven patients were randomized in Japan (pembrolizumab plus chemotherapy,n= 61; placebo plus chemotherapy,n= 26), 66 (76%) had PD‐L1 CPS ≥1, and 28 (32%) had PD‐L1 CPS ≥10. Median time from randomization to data cutoff (June 15, 2021) was 44.7 (range, 37.2–52.9) months in the ITT population. Hazard ratios (HRs; 95% CI) for OS were 0.36 (0.14–0.89), 0.52 (0.30–0.91), and 0.46 (0.28–0.77) in the PD‐L1 CPS ≥10, PD‐L1 CPS ≥1, and ITT populations, respectively. HRs (95% CI) for PFS were 0.52 (0.20–1.34), 0.61 (0.35–1.06), and 0.64 (0.39–1.05). Grade 3 or 4 treatment‐related adverse events occurred in 85% of patients in each group (no grade 5 events). Consistent with the global population, pembrolizumab plus chemotherapy tended to show improvements in OS and PFS with manageable toxicity versus placebo plus chemotherapy in Japanese patients and supports this combination in this setting.