Background Anti-NMDAR encephalitis is an emerging differential diagnosis of first episode and persistent psychosis in the psychiatric community, as clinical manifestations include psychiatric symptoms, cognitive dysfunction, seizures, decreased consciousness, and dyskinesias. This disease is associated with extreme delta brush (EDB), but the significance and temporal course of this EEG pattern still needs to be determined. Herein, we report a case of anti-NMDAR encephalitis with persistent psychosis associated with EDB occurrence on multiple occasions during a 5-year disease course. Case presentation A 15-year-old girl was diagnosed with anti-NMDAR encephalitis and treated with progressive improvement. Four years after initial manifestations, an EDB pattern was seen on electroencephalogram (EEG) without new neurological symptoms. She had residual symptoms of episodic auditory hallucinations and impulsivity. One year later, the patient had a recurrence of neurological symptoms (seizures, dyskinesias and impaired attention), persisting with EDB on EEG. Clinical symptoms and EDB resolved after second-line treatment with rituximab. Conclusion We describe the first case of persistent psychosis in anti-NMDAR encephalitis associated with extreme delta brush on multiple EEGs on prolonged follow-up. Electroencephalographic patterns such as EDB may serve as markers of residual disease activity, including psychiatric symptoms. Further studies with prolonged EEG monitoring are needed to better understand these findings.