[Abstract] Introduction: Space-occupying lesions in the stomach are common diseases in gastroenterology. The diagnosis often needs the support of pathological results, and the prognosis of different masses is very different. Patient concerns: A 49-year-old man presented with abdominal distension and loss of appetite for one month as the main symptoms. Gastroscopy revealed a space occupying lesion in the pylorus of the antrum. Interventions:The mass was removed by gastroscopy resection with high-frequency electrocautery. Diagnosis: After electroresection, pathological and immunohistochemical examination showed that it was “ectopic duodenal mucosa with adenomatous hyperplasia in the stomach”. This disease has not been reported. Outcomes: After short-term follow-up and endoscopic review of this patient, we concluded that this disease is an extremely rare gastric tumor with benign growth and good prognosis. Conclusion: This case helps us to recognize the pathology and treatment of this disease, which is helpful for the differential diagnosis of other gastric space-occupying diseases.