Wernicke's encephalopathy (WE) is a condition resulting from thiamine deficiency that typically presents with acute neurologic symptoms including ataxia, eye movement disorders, and altered mental status. Though classically seen in patients with alcohol use disorder, it can also occur as a complication of bariatric surgery and gastrointestinal cancers. Here, we present a patient with a history of gastric band surgery and an intact alimentary tract. She presented with acute, intractable vomiting and epigastric abdominal pain, incompletely relieved by deflating her gastric band, and was found to have duodenal adenocarcinoma causing partial duodenal obstruction. She was then found to have binocular diplopia, horizontal nystagmus, dizziness, reduced proprioception, and pins-and-needles numbness in her bilateral lower extremities, and there was concern for gait instability; thus, WE was suspected. The patient was treated with high-dose thiamine repletion, and her symptoms resolved shortly thereafter. WE is rare in patients who have undergone gastric band surgery, and to our knowledge, this is the first case of WE in a patient with concurrent duodenal adenocarcinoma. This case illustrates that patients with a history of bariatric surgery may be more susceptible to developing WE in the face of a new gastrointestinal insult, such as duodenal cancer.