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We present a unique case of metastatic metaplastic breast carcinoma responding dramatically to immunochemotherapy. A 46‐year‐old Japanese woman with primary metaplastic carcinoma of the breast, which was immunohistochemically confirmed to be triple‐negative breast cancer, underwent radical surgery, followed by adjuvant chemotherapy with an anthracycline and a taxane. Since multiple lung metastases were detected two months post‐chemotherapy and the primary site was shown to be PD‐L1‐positive, the immune checkpoint inhibitor (ICI) pembrolizumab plus gemcitabine/carboplatin was initiated. While the treatment was discontinued after 15 days due to suspected drug‐induced pneumonitis, the lung metastases significantly shrank with no development of new lesions for three months. The patient remained alive as of approximately 15 months after the recurrence date. This case highlights the potential of immunochemotherapy in treating metaplastic breast carcinomas. [ABSTRACT FROM AUTHOR]