Immune checkpoint inhibitors (ICIs) including anti-cytotoxic T-lymphocyte-associated protein 4 (anti-CTLA4) and anti-programmed death cell protein 1 (anti-PD1) have extended patient survival benefit and revolutionized cancer treatment. As ICIs rely on immune regeneration to eliminate tumor cells, they can also lead to an imbalance of immune reactions often called immune-related adverse events (irAEs). Rare irAEs such as ocular or cardiac toxicity or vasculitis are seen in less than 1% of patients receiving ICIs. Immune-related cystitis remains a rare occurrence. Herein, we describe a patient with extensive-stage small cell lung cancer (SCLC) and a history of syphilis with a complete response to second-line treatment using nivolumab plus paclitaxel who complained of urinary irritation symptoms. At biopsy, we found infiltration of CD3+ and CD8+ lymphocytes in the urothelium. Although there are reports describing immune-related cystitis in cancer patients, our case has comprehensive pathological confirmation and a differentiation diagnosis. In this report, we review other cases to elucidate clinical characteristics and discuss suitable management of this rare irAE.