Wernicke encephalopathy (WE), also Wernicke's encephalopathy, or wet brain is the presence of neurological symptoms caused by biochemical lesions of the central nervous system after exhaustion of B-vitamin reserves, in particular thiamine (vitamin B₁). The condition is part of a larger group of thiamine deficiency disorders that includes beriberi, in all its forms, and alcoholic Korsakoff syndrome. When it occurs simultaneously with alcoholic Korsakoff syndrome it is known as Wernicke–Korsakoff syndrome.

Classically, Wernicke encephalopathy is characterised by a triad of symptoms: ophthalmoplegia, ataxia, and confusion. Around 10% of patients exhibit all three features, and other symptoms may also be present. While it is commonly regarded as a condition peculiar to malnourished people with alcohol misuse, it can be caused by a variety of diseases. It is treated with thiamine supplementation, which can lead to improvement of the symptoms and often complete resolution, particularly in those where alcohol misuse is not the underlying cause. Often other nutrients also need to be replaced, depending on the cause; however medical literature notes the importance of managing the condition in a timely fashion to avoid worsening symptoms.

Wernicke encephalopathy may be present in the general population with a prevalence of around 2%, and is considered underdiagnosed; probably, many cases are in patients who do not have commonly-associated symptoms.