BACKGROUND Enterocolitis is an immune-related adverse event associated with nivolumab treatment. Although intravenous corticosteroids and infliximab are recommended as a first-line and second-line therapy, respectively, there is no established treatment for severe enterocolitis that is refractory to these drugs. CASE REPORT A 62-year-old male with non-small cell lung cancer, with multiple brain metastasis, received nivolumab as the eighth-line chemotherapy for his disease. A few days after nivolumab administration, grade 2-3 enterocolitis developed in the patient. The enterocolitis improved to grade 1 after careful observation; however, it was aggravated to grade 3 after resuming nivolumab treatment. After cessation of nivolumab, 3.3 mg of intravenous dexamethasone and 40 mg of methylprednisolone were administered for 16 days and subsequently 30-60 mg of oral prednisolone was administered for 50 days, with little improvement in the patient's colitis. A second-line treatment with 5 mg/kg of infliximab was twice attempted, but the patient had persistent diarrhea. Therefore, 50 mg of oral cyclosporine was started as a third-line therapy. Three days after the start of cyclosporine, the number of diarrhea events decreased, with resolution 2 weeks after cyclosporine administration. CONCLUSIONS Oral cyclosporine treatment can be a third-line therapy for enterocolitis associated with immune-related adverse events.