This report describes a case of Hashimoto encephalopathy in an 11-year-old girl. She presented with features typical of this disorder including encephalopathy, seizures, and neuropsychiatric symptoms. Diagnosis was supported by an elevated thyroid-stimulating hormone level, a low levo-thyroxine level, and positive results for antithyroperoxidase antibody. Her response to typical treatment with levo-thyroxine was incomplete, requiring additional therapy with valproic acid and methylprednisone. Her course was further complicated by the development of acute psychosis. Treatment with risperidone was correlated with resolution of her psychosis and improvement in neuropsychiatric symptoms. Response to antipsychotic therapy has not previously been described in the pediatric population with Hashimoto encephalopathy. This case highlights the need for guidelines for the management of this rare disorder.