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Rationale: Pulmonary pleomorphic carcinoma is a rare tumor with a poor prognosis and has no standard chemotherapy. We herein report a case of small intestinal metastasis of pulmonary pleomorphic carcinoma that resulted in intestinal bleeding and was successfully treated with pembrolizumab monotherapy. Patient concerns: A 54-year-old man with a history of pulmonary pleomorphic carcinoma resection was referred to our hospital due to a 1-month history of a fever and general fatigue. Diagnosis: Laboratory investigation revealed microcytic anemia. Hematochezia was also noted after admission. Computed tomography (CT) and positron emission tomography (PET)/CT at the time of this admission revealed intraperitoneal masses alongside the small intestine with no significant ascites. Interventions: Pembrolizumab (400 mg/body) was introduced as the first-line chemotherapy. Outcomes: By the 15th day after the initial pembrolizumab administration, the fever had disappeared, and the intraperitoneal masses were markedly reduced. Hematochezia had also disappeared, and he no longer needed to receive blood transfusions. Lessons: To our knowledge, this is the first report in which small intestinal metastasis of pulmonary pleomorphic carcinoma was successfully controlled by pembrolizumab monotherapy. Immune checkpoint inhibitors may be promising therapeutic agents against pulmonary pleomorphic carcinoma.