Gastrointestinal beriberi is an underrecognized yet important manifestation of thiamine deficiency that may occur even in individuals without a history of chronic alcohol use. It often presents with nonspecific upper gastrointestinal symptoms, leading to diagnostic delay. The case of a woman in her 60s with a history of distal gastrectomy who developed persistent anorexia and intractable diarrhea over four months, resulting in multiple hospitalizations, is presented. Despite comprehensive evaluations, including blood tests, CT scans, stool analyses, endoscopy, and biopsies, no definitive cause was identified. Furthermore, she was not taking any medications that might have contributed to the diarrhea. She had no recent alcohol misuse, neurological deficits, or lactic acidosis, which are classical features typically associated with thiamine deficiency. Given her surgical history and ongoing symptoms, thiamine deficiency was suspected. A serum thiamine level measured on hospital day one was low at 14 ng/ml (reference range 26-58), which was consistent with thiamine deficiency. Oral thiamine led to the rapid resolution of symptoms within one week, and we therefore diagnosed thiamine deficiency. This case highlights gastrointestinal beriberi as an early, isolated manifestation. Diarrhea was the predominant symptom, a less-reported feature. Early recognition and treatment may prevent serious complications such as Wernicke's encephalopathy, even without classic risk factors.