Viral encephalitis is inflammation of the brain parenchyma, called encephalitis, by a virus. The different forms of viral encephalitis are called viral encephalitides. It is the most common type of encephalitis and often occurs with viral meningitis. Encephalitic viruses first cause infection and replicate outside of the central nervous system (CNS), most reaching the CNS through the circulatory system and a minority from nerve endings toward the CNS. Once in the brain, the virus and the host's inflammatory response disrupt neural function, leading to illness and complications, many of which frequently are neurological in nature, such as impaired motor skills and altered behavior.

Viral encephalitis can be diagnosed based on the individual's symptoms, personal history, such as travel history, and different clinical tests such as histology, medical imaging, and lumbar punctures. A differential diagnosis can also be done to rule out other causes of the encephalitis. Many encephalitic viruses often have characteristic symptoms of infection, helping to aid diagnosis. Treatment is usually supportive in nature while also providing antiviral drug therapy. The primary exception to this is herpes simplex encephalitis, which is treatable with acyclovir. Prognosis is good for most individuals who are infected by an encephalitic virus but is poor among those who develop severe symptoms, including viral encephalitis. Long-term complications of viral encephalitis typically relate to neurological damage, such as experiencing seizures, memory loss, and intellectual impairment.

Encephalitic viruses are typically transmitted either from person-to-person or are arthropod-borne viruses, called arboviruses. The young and the elderly are at the highest risk of viral encephalitis. Many cases of viral encephalitis are not identified either because of lack of testing or mild illness, and serological surveys indicate that asymptomatic infections are common. Various ways of preventing viral encephalitis exist, such as vaccines that are either in standard vaccination programs or which are recommended when living in or visiting certain regions, and various measures aimed at preventing mosquito, sandfly, and tick bites in order to prevent arbovirus infection.