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IgG4‐related diseases are adverse events that occur after receiving treatment with immune checkpoint inhibitors (ICI). This study reports the first case of IgG4‐related retroperitoneal fibrosis after the administration of chemotherapy with nivolumab and ipilimumab (NI therapy). An 80‐year‐old man developed lower abdominal pain eight months after NI therapy was initiated. Although the primary lesion maintained its reduced size on computed tomography, there was an increase in the soft tissue shadows intensity around the abdominal aorta, bladder, and seminal vesicles, suggesting retroperitoneal fibrosis. Blood tests showed elevated IgG4 levels. Computed tomography‐guided biopsy of the retroperitoneum showed B cell‐dominant lymphocyte infiltration consistent with IgG4‐related retroperitoneal fibrosis and characteristic CD8‐positive lymphocyte infiltration, suggestive of the involvement of cytotoxic T cells. Based on the clinical, imaging, and pathological findings, the patient was diagnosed with IgG4‐related retroperitoneal fibrosis due to ICI. Immunotherapy discontinuation alone did not result in improvement; therefore, steroid therapy was initiated. In clinical practice, IgG4‐related retroperitoneal fibrosis can occur as an immune‐related adverse event when administering anti‐PD‐1 and anti‐CTLA‐4 antibodies for cancer immunotherapy. Early steroid therapy could be effective in controlling this immune‐related adverse event. [ABSTRACT FROM AUTHOR]