Postgastrectomy diarrhea is often attributed to dumping syndrome or functional changes; however, exocrine pancreatic insufficiency (EPI) from anatomical and physiological alterations may be an underrecognized cause of malabsorption and nutritional decline. Because EPI symptoms are often nonspecific, it may remain undiagnosed and lead to progressive malnutrition if untreated. This case report describes severe EPI identified via nutrition-focused assessment in a patient with persistent diarrhea after Billroth II gastrectomy, and the clinical response to pancreatic enzyme replacement therapy (PERT). A patient with a history of subtotal gastrectomy with Billroth II reconstruction for gastric cancer presented with chronic diarrhea, steatorrhea, weight loss, and hypoalbuminemia. Repeated endoscopic and radiologic evaluations identified no structural cause of diarrhea. Comprehensive nutrition-focused assessment indicated fat malabsorption, and fecal pancreatic elastase was markedly reduced (23.8 µg/g), confirming severe EPI. PERT with pancreatin containing 25,000 units of lipase (Norzyme) was initiated with meals. Posttreatment, steatorrhea resolved and bowel movements normalized without dietary fat restriction. Serum albumin levels subsequently normalized, and body weight returned to the normal range, indicating improved nutritional status. This case emphasizes the clinical value of nutrition-focused assessment in identifying treatable causes of malabsorption, such as EPI, in patients with persistent postgastrectomy diarrhea.