Wernicke's encephalopathy is a reversible but potentially critical disease caused by thiamine deficiency. Most patients complain of symptoms such as ophthalmoplegia, ataxia and confusion. Heavy alcohol drinking is commonly associated with the disease, but other clinical conditions also can provoke it. In pregnant women, hyperemesis gravidarum can lead to the depletion of body thiamine due to poor oral intake and a high metabolic demand. We report a case of Wernicke's encephalopathy following hyperemesis gravidarum in a 36-year-old female at 20 weeks of pregnancy, who visited our hospital because of shock with vaginal bleeding. This case suggests that although the initial presentation may include atypical symptoms (e.g., shock or bleeding), Wernicke's encephalopathy should be considered, and thiamine replacement should be performed in pregnant women with neurologic symptoms and poor oral intake.