Hashimoto's encephalopathy is a rare, chronic relapsing and remitting encephalopathy associated with auto-antibodies against thyroid components. Clinically it is characterized by tremor, myoclonus, stroke-like episodes, seizures, impairment of consciousness, and dementia. We described here a 13-year-old boy who presented with generalized tonic clonic seizures associated with Hashimoto's encephalopathy. After corticosteroid treatment, his neurological features improved dramatically. As a result, Hashimoto's encephalopathy should be considered in cases of unexplained encephalopathy and seizures in pediatric patients.