Here we report the case of a patient with advanced gastric cancer who presented with duodenal intramural metastasis based on the pathological results after surgery. The patient was 78-year-old female, who was referred to our department for further evaluation and treatment of upper abdominal pain. An upper gastrointestinal series demonstrated a tumor occupying the lesser curvature of the gastric body. Biopsy specimens from the tumor demonstrated moderately to poorly differentiated adenocarcinoma. A computed tomography scan showed thickening of the gastric wall and swelling of the regional lymph nodes. The patient underwent distal gastrectomy and D2 lymph node dissection for gastric cancer. A histopathological examination disclosed that the gastric tumor was poorly differentiated adenocarcinoma with severe lymphatic permeation and also demonstrated the other poorly differentiated adenocarcinoma occupying the part of the muscularis propria layer of the duodenum. The gastric tumor was not contiguous with the duodenal tumor, and the duodenal cancer cells had the same pathological characteristics as the primary gastric cancer cells; therefore, we diagnosed the duodenal tumor as an intramural metastasis from gastric cancer. The patient's disease was staged as pT4aN3bM1, Stage Ⅳ according to the TNM classification. We report this rare case along with a discussion of the literature.