The clinical manifestation and nuclear imaging findings in a 15-year-old boy with anti-N-methyl-d-aspartate receptor (NMDAR) encephalitis are described in this case report. The previously healthy patient presented with new onset hallucinations, seizure, and within a week, his mental status rapidly deteriorated to nonverbal with oro-lingual-facial dyskinesias. An extensive laboratory work-up for encephalopathy was negative. Repeated brain magnetic resonance imaging (MRI) studies were normal. On day 26 of admission, nuclear imaging using fluorodeoxyglucose positron emission tomography (FDG-PET) showed global hypometobolism with a prominent focally intense hypermetabolic lesion in the right cerebellar cortex. Diagnosis of anti-NMDAR encephalitis was confirmed with quantitative serology. The patient showed clinical signs of improvement after 2 courses of intravenous immunoglobulin therapy over 4 weeks. On day 46, repeat brain FDG-PET showed overall improvement but in contrast to the previous, the right cerebellar cortex showed focal hypometabolism. This is the first reported case of such findings using FDG-PET in anti-NMDAR encephalitis.