Here, we report a case of Hashimoto's encephalopathy (HE) mimicking Creutzfeldt-Jakob disease (CJD). A 57-year-old man was admitted to our hospital for status epilepticus. He had gradually presented personality change over the last two years. On admission, he was in state of akinetic mutism. He exhibited seizures on the left side of his body, including the face, and intermittent myoclonic movement. Routine laboratory tests showed no abnormalities, including thyroid functions. An EEG study showed typical periodic synchronous discharge (PSD). Brain MRI showed high-intensity areas in the bilateral frontal cortex, thalamus, and right insula on diffusion-weighted imaging (DWI). So, initially, sporadic CJD was suspected. However, there were no abnormalities in the caudate or putamen on MRI. Anti-TG and anti-TPO antibodies, as well as anti-NAE antibody were all positive. He was administered methylpredonisolone pulse therapy. Subsequently, his consciousness levels and EEG and MRI findings markedly improved. So, he was finally diagnosed with HE. HE should be considered in patients with PSD on EEG, even if the patients have typical MRI abnormalities of CJD. Anti-thyroid antibodies should be examined in such patients.