Immune checkpoint inhibitors have been critical in the treatment of advanced malignancies in recent years. Encephalitis caused by atezolizumab is an uncommon immune-related adverse event. The case of a 65-year-old female diagnosed with encephalitis closely associated with atezolizumab medication for metastatic advanced breast cancer is presented in the current study. Following a fourth atezolizumab dose 10 days previously, the patient fell into a deep coma. Initial brain magnetic resonance imaging revealed multiple patchy T2 hyperintensities in the bilateral cerebellar hemisphere, vermis of the cerebellum, bilateral frontal lobe, temporal lobe, parietal lobe and occipital cortex. Meanwhile, there were aberrant signs on diffusion-weighted imaging. The diagnosis of atezolizumab-induced encephalitis seemed probable after ruling out other possible causes of encephalitis. Subsequently, the condition of the patient worsened and there were indications of cardiac and respiratory arrest. Chest compressions were provided immediately, as well as a balloon mask for assisted ventilation, a medication boost, stimulated breathing and other symptomatic therapy. The patient's vital signs temporarily stabilised after this series of rescue measures. The patient refused further therapy and insisted on being discharged, and died a few days after being discharged from the hospital. In this case, the patient's encephalitis symptoms associated with atezolizumab were not as typical as previously documented. The patient's condition swiftly deteriorated to heartbeat apnea, and steroid pulse therapy was not received in a timely manner, resulting in an unfavourable outcome.