IntroductionAlthough immune checkpoint inhibitors offer significant therapeutic benefits to patients with advanced cancer, they can also cause a variety of immune‐related adverse events. As immune checkpoint inhibitors are being widely used, rare immune‐related adverse events are being reported.Case presentationA 70‐year‐old man with advanced salivary duct carcinoma was treated with pembrolizumab following radiotherapy. After receiving two doses of pembrolizumab, the patient experienced symptoms such as micturition pain and hematuria. Immune‐related cystitis was suspected, and the patient underwent a bladder biopsy and bladder hydrodistension. Histological analysis revealed non‐neoplastic bladder mucosa with CD8‐positive lymphocyte‐dominant inflammatory cell infiltration, consistent with immune‐related cystitis. The patient's bladder symptoms improved postoperatively without steroid administration.ConclusionAlthough steroids are commonly administered to treat immune‐related adverse events, bladder hydrodistension may be a promising treatment option for immune‐related cystitis to avoid administration of steroids, which may impair the therapeutic effect of immune checkpoint inhibitors.