We herein report a case of phlegmonous gastritis secondary to superior mesenteric artery syndrome. An 80-year-old woman visited the hospital emergency department with the chief complaints of epigastric pain and vomiting. She was hospitalized urgently following the diagnosis of superior mesenteric artery syndrome based on abdominal computed tomography findings. Conservative therapy was not effective, and phlegmonous gastritis was diagnosed based on the findings of upper gastrointestinal endoscopy and biopsy performed on the 12th day of the disease. Undernutrition and reduced physical activity were observed on hospital admission, and proactive nutritional therapy with enteral nutrition was started. An upper gastrointestinal series, performed approximately 1 month later, confirmed the persistence of strictures and impaired gastric emptying. Because conservative therapy was unlikely to improve oral food intake, open total gastrectomy was performed on the 94th day of the disease. Examination of surgically resected specimens revealed marked inflammation and fibrosis, especially in the body of the stomach. Following a good postoperative recovery, the patient was able to commence oral intake and left our hospital on foot approximately 1 month after surgery.