Wernicke's encephalopathy is caused by severe thiamine deficiency; it is mostly observed in alcoholic patients. We report the case of a 28-year-old woman, at 17 weeks of gestational age, with severe hyperemesis gravidarum. She presented with disturbance of consciousness, nystagmus, ophthalmoplegia, and ataxia. The resonance magnetic imagery showed bilaterally symmetrical hyperintensities of thalamus and periaqueductal area. The case was managed with very large doses of thiamine. The diagnosis of Wernicke's encephalopathy was confirmed later by a low thiamine serum level. The patient was discharged home on day 46 with mild ataxia and persistent nystagmus. Wernicke's encephalopathy is a rare complication of hyperemesis gravidarum. It should be diagnosed as early as possible to prevent long-term neurological sequela or death. Thiamine supplementation in pregnant women with prolonged vomiting should be initiated, especially before parenteral dextrose infusion. Early thiamine replacement will reduce maternal morbidity and fetal loss rate.