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Introduction: Immune checkpoint inhibitors such as programmed cell death/-ligand 1 inhibitor and cytotoxic T-lymphocyte-associated antigen-4 inhibitors have been widely used for various advanced malignancies. The mechanism of action for these inhibitors is the improvement of antitumor immunity via T-cell modulation. On the contrary, immune-related adverse events such as autoimmune colitis might arise in association with T-cell activation. Upper gastrointestinal adverse events related to pembrolizumab have rarely been reported., Case Presentation: A 72-year-old man underwent laparoscopic radical cystectomy for muscle-invasive bladder cancer (pT2N0M0). Multiple lymph node metastases appeared in the paraaortic region. First-line chemotherapy comprising gemcitabine and carboplatin failed to stop disease progression. After the administration of pembrolizumab as second-line treatment, the patient showed symptomatic gastroesophageal reflux disease. Esophagogastroduodenoscopic biopsy of the gastric body showed severe lymphoplasmacytic and neutrophilic infiltration., Conclusion: We present acute gastritis related to pembrolizumab. Early eradication therapy may be able to control immune checkpoint inhibitor-related gastritis., Competing Interests: The authors declare no conflicts of interest., (© 2022 The Authors. IJU Case Reports published by John Wiley & Sons Australia, Ltd on behalf of Japanese Urological Association.)