In infectious enteritis, the affected site differs depending on the organism causing the disease, and based on this feature, it is possible to estimate the causative organisms of infectious enteritis. However, there have been few reports of infectious enteritis mainly affecting the duodenum. This report describes a case of duodenal enteritis that was suspected to be associated with Legionella infection. The patient was a 66-year-old man. He was febrile for 5 days and had watery diarrhea for 3 days before admission, and he presented to our hospital when the symptoms did not improve. He was hypoxemic at presentation, and a CT scan on admission revealed nonsegmental ground-glass opacities in both lung fields and thickening of the intestinal wall, mainly in the duodenum. Blood tests and chest imaging findings led to suspicion of Legionella pneumonia. Urine specimens were submitted for Legionella antigen testing and were found to be positive, confirming the diagnosis. Subsequent antimicrobial therapy resulted in improvement of the diarrhea symptoms and duodenal wall thickening, and the patient was discharged on hospital day 19. This case shows that Legionella pneumonia can manifest as abdominal symptoms. The possibility of Legionella pneumonia should thus be considered in patients with duodenitis.