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Encephalitis is an inşammation of the brain. Most often, it is caused by several types of viruses especially the herpes viruses. People with the disease mostly have şu-like symptoms, such as fever, sore throat, cough, and malaise. People with severe encephalitis usually have some change in consciousness, ranging from mild confusion to coma. Herpes viruses selectively impact the frontal and temporal lobes. In this case, we present a patient admitted with the clinical picture of delirium and diagnosed as herpes simplex encephalitis. A 58-year-old female patient was admitted to our hospital psychiatric clinic with insomnia, feeling discomfort, decreased appetite, feeling of fear for no reason, always using the same words, speaking meaninglessly, pausing and strolling aimlessly. The symptoms that occurred after psychosocial stress were started two days ago. In mental examination, her orientation was partially complete, but the cooperation was limited. Her amount of speech was decreased and the reaction time was prolonged in the conversation. The concentration and the attention of the patient had been decreased. We suspected over her diagnose because of not having any psychiatric disorder before and her atypical clinical presentation. We referred the patient to our emergency department for differential diagnosis to check if, there is another medical condition. In the emergency department, she had been examined again. She had fever and her leukocyte count was increased. Computerized tomography scan of the brain (without contrast) was unremarkable for mass or bleed. The cranial MRI showed left temporal lobe enhancement in the amygdala and insular cortex, which was consistent with herpes encephalitis. Lumbar puncture was performed. The results were confirmed our diagnosis. After these findings, antiviral medication with intravenous acyclovir had been started immediately. The patient hospitalized in infectious diseases and clinical microbiology service with the diagnosis of herpes encephalitis. PCR testing of the spinal şuid confirmed the presence of Herpes simplex virus type 1.She had no fever after two weeks. Her intravenous medication had been completed to three weeks. She was discharged with noneurologic sequel and psychiatric symptoms. She returned her healthy living. Delirium is a syndrome that presents as severe confusion, disorientation, cognitive deficits and altered sleep-wake cycle, developing with relatively rapid onset and şuctuating in intensity. Delirium itself is not a disease, but rather a clinical syndrome, which results from an underlying disease. Many precipitating and predisposing factors can trigger delirium. Identification of underlying causes and risk factors is very essential for early diagnosis and treatment. We aimed to emphasize that further investigations are very important for the patients admitted with atypical şuctuating cognitive symptoms. Morbidity may be prevented by early recognition.