Herpes simplex virus (HSV) is a recognized cause of gastrointestinal infection in immunodeficient patients. Although a few cases of HSV gastritis and colitis in immunocompromised patients have been reported, there are no reports of HSV duodenitis in patients with Crohn's disease (CD). A 74-year-old female was admitted with general weakness and refractory epigastric pain. She had been diagnosed with CD three years ago. Esophagogastroduodenoscopy (EGD) revealed diffuse edematous and whitish mucosa with multiple erosions in the duodenum. Considering the possibility of viral co-infection, cytomegalovirus (CMV) immunohistochemical staining, PCR, and cultures of duodenal biopsies were performed, all of which were negative with the exception of the isolation of HSV in culture. After administration of intravenous acyclovir for 1 week, follow-up EGD showed almost complete resolution of the lesions and the patient's symptoms improved. In CD patients with refractory gastro-intestinal symptoms, HSV, as well as CMV, should be considered as a possible cause of infection, so that the diagnosis of viral infection is not delayed and the appropriate antiviral treatment can be initiated.