Adenocarcinoma is the most common histological type of gastric tumor. Gastric tumor arising from collision of an adenocarcinoma with a neuroendocrine carcinoma is extremely rare. Moreover, this uncommon gastric collision tumor in our case had prolapsed into the duodenum. A 77-year-old woman was admitted to our hospital complaining of vomiting and severe weight loss. Abdominal X-ray showed gastric distension, and computed tomography revealed a duodenal giant mass spreading from the bulb to the horizontal part of the duodenum. Upper gastrointestinal endoscopy was not helpful in confirming the diagnosis of the tumor. We suspected duodenal malignant tumor and performed laparotomy. The operative findings indicated that the gastric antrum was deeply invaginated into the duodenum because of the gastric tumor. Partial resection of the stomach and duodenum was performed because the tumor was irreducible. Intraoperative diagnosis of the frozen section was well-differentiated adenocarcinoma and undifferentiated carcinoma. Additional distal gastrectomy with lymphadenectomy was performed. We herein report the first case of gastroduodenal intussusception caused by a gastric collision tumor consisting of well-differentiated adenocarcinoma and poorly differentiated neuroendocrine carcinoma.