A 59-year-old man on hemodialysis was referred to our hospital with cough and chest pain, and a left upper lobe mass was detected on chest CT. He was diagnosed with Stage IVB high-grade lung mucoepidermoid carcinoma. PD-L1 evaluation was performed by immunohistochemistry and the PD-L1 tumor proportion score was more than 75%. Therefore, he received pembrolizumab as a first-line treatment, resulting in a partial response. He received a total of 22 courses of treatment, which were discontinued due to the onset of atrial fibrillation. He maintained a partial response for about 6 months after the cessation of treatment with pembrolizumab. Lung mucoepidermoid carcinoma is rare, and effective treatment has not yet been established for advanced-stage cases. This case suggests that pembrolizumab could be effective for the treatment of lung mucoepidermoid carcinoma with high PD-L1 expression.