Background: Immune checkpoint inhibitor (ICI)-related myocarditis is an uncommon but potentially fatal immune-related adverse event. Corticoid-resistant myocarditis induced by ICI is an important therapeutic challenge. Case summary: Here, we present a case of steroid-refractory ICI-related myocarditis and myositis treated with abatacept and mycophenolate mofetil (MMF). A 57-year-old male with metastatic renal cell carcinoma was diagnosed with immune-related myocarditis and myasthenia gravis-like myositis after first dose of combination ICIs with nivolumab (anti-programmed cell death-1) plus ipilimumab (anti-cytotoxic T-lymphocyte-associated antigen-4). Twelve days after ICI he was admitted to the hospital due to palpitations, headache, and pain in the extremities. Laboratory findings revealed elevated inflammatory markers and cardiac enzymes. Electrocardiogram showed first-degree atrioventricular (AV) block and right bundle branch block which developed into complete heart block within 48 h. Because of clinical and paraclinical deterioration despite immediate initiation of methylprednisolone abatacept and MMF was added. Following, gradual subjective improvement and termination of arrhythmia led to discharge of the patient from the hospital 6 weeks after the introduction of ICI. Discussion: The key treatment of ICI-related myocarditis is glucocorticoid. For steroid-refractory myocarditis supplementary immune suppressive agents are recommended. Yet, data still relies on case reports and case series, due to lack of prospective studies. In this case, the use of abatacept and MMF led to resolution of steroid-resistant ICI-related myocarditis and myositis.