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Abstract Background Immune checkpoint inhibitors have recently become the standard of care in the first‐line treatment of extensive‐stage small cell lung cancer. Although immune‐related adverse events have been reported to influence prognosis in non‐small cell lung cancer patients, few studies have investigated the prognostic value of immune‐related adverse events in small cell lung cancer patients. In this study, we evaluated the prognosis of patients who developed immune‐related adverse events after first‐line treatment with immune checkpoint inhibitor‐based chemotherapy for extensive‐stage small cell lung cancer. Methods We enrolled 90 patients with extensive‐stage small cell lung cancer who received immune checkpoint inhibitor‐based chemotherapy as first‐line treatment from September 2019 to December 2022 in six hospitals in Japan. The patients were categorized into groups with and without immune‐related adverse events. Results There were 23 patients with and 67 without immune‐related adverse events. Seventeen patients had grade 1–2 immune‐related adverse events, and nine (including overlapping cases) had grade ≥3. The most frequent immune‐related adverse event was a skin rash. The median survival time was 22 months in patients with immune‐related adverse events and 9.3 months in patients without immune‐related adverse events. The hazard ratio was 0.40 (95% confidence interval: 0.19–0.83, p = 0.013). Conclusions The results of this study show that immune‐related adverse events are associated with improved survival outcomes in patients with extensive‐stage small cell lung cancer.