Background and Objectives <p>Pembrolizumab has been approved for the immunotherapy of programmed death ligand 1 (PD-L1)-positive triple-negative breast cancer (TNBC) based on the KEYNOTE-355 trial. However, cost-effectiveness evidence is limited. The purpose of this study was to evaluate the cost-effectiveness of pembrolizumab plus chemotherapy compared with chemotherapy alone for patients with PD-L1-positive inoperable or metastatic TNBC from a Japanese healthcare perspective.</p> Methods <p>The cost-effectiveness analysis was performed for pembrolizumab, of which the drug price was determined at 214,498 Japanese yen (JPY), or 1631 US dollars (USD) (1 USD = 131.5 JPY) for KEYTRUDA^® (100 mg), using a partition survival model based on the KEYNOTE-355 trial subgroup analysis in Japan<b>.</b> The comparison was made using quality-adjusted life years (QALYs) and the incremental cost-effectiveness ratio (ICER). One-way deterministic and probabilistic sensitivity analyses (PSA), which evaluate the impact of parameter uncertainty, were performed to assess the robustness and calculate the acceptable probability, defined as the probability of the ICER being below the willingness-to-pay (WTP).</p> Results <p>Pembrolizumab plus chemotherapy provided an additional 0.676 QALYs at an incremental cost of 8,503,072 JPY. The ICER for pembrolizumab plus chemotherapy compared with conventional chemotherapy was 12,577,178 JPY (95,644 USD) per QALY. The ICER per QALY was below the willingness-to-pay threshold of 15,000,000 JPY. PSAs revealed that the acceptable probability was 83.9% at 15,000,000 JPY.</p> Conclusions <p>The pembrolizumab plus chemotherapy is likely to be a cost-effective option compared with conventional chemotherapy for patients with PD-L1-positive inoperable or metastatic TNBC in a Japanese medical environment from a healthcare system.</p>