Brunner's gland hamartoma is a submucosally localized and rarely encountered benign condition. Majority of cases are asymptomatic. Generally, it is diagnosed incidentally during endoscopic examinations of the upper gastrointestinal system (GIS) indicated for other reasons. It may sometimes present with symptoms of GIS bleeding or intestinal obstruction. In this paper, we present a 56-year-old female who was admitted to the emergency department with the complaints of nausea-vomiting and black-colored stool for two days. In upper GIS endoscopy, which was performed due to a pre-diagnosis of upper GIS bleeding, a submucosal mass was detected in the duodenum. The decision of surgery was taken for the patient who had an abdominal computed tomography demonstrating a duodenal mass. As a 4-cm polypoid mass was detected in duodenotomy performed after Kocher manoeuvre, mass excision was performed. The diagnosis of Brunner's gland hamartoma was established based on the pathological analysis of the surgical specimen. The patient, who had no problem, but atelectasis in the postoperative follow-up, was discharged on the postoperative day seven.