ABSTRACT Anti‐N‐methyl‐D‐aspartate receptor encephalitis is a severe, potentially treatable, disorder and prognosis depends on early recognition and prompt immunotherapy. We report a case of anti‐N‐methyl‐D‐aspartate receptor encephalitis with atypical age and gender, and a characteristic electroencephalographic pattern that supported the diagnosis. A 66‐year‐old male presented with psychiatric disturbances and focal seizures with alteration of consciousness, and progressed to a state of akinetic mutism. Auxiliary tests were negative or non‐specific for anti‐NMDAR encephalitis. Electroencephalographic monitoring revealed a unique pattern; the extreme delta brush. The patient improved with immunotherapy and was asymptomatic at six months of follow‐up. Ancillary testing was positive for anti‐N‐methyl‐D‐aspartate receptor antibodies. Extreme delta brush is a recently described electroencephalographic pattern presenting in only one third of patients with anti‐N‐methyl‐D‐aspartate receptor encephalitis. The identification of this pattern, as in our case, may guide early diagnosis and treatment of anti‐N‐methyl‐D‐aspartate receptor encephalitis.