Background <p>Gastric cancer is the fourth leading cause of cancer death worldwide.</p> Case summary <p>We reported a 64-year-old male patient with paroxysmal dull pain around the periumbilical region after meals. After admission, the patient underwent contrast-enhanced computed tomography (CT) scans of the upper and lower abdomen, which revealed marked thickening of the gastric antrum and multiple enlarged lymph nodes around the gastric antrum and in the retroperitoneum. Esophagogastroduodenoscopy revealed possible tumor lesions at the cardia, gastric body, duodenal bulb, and descending part of the duodenum. Colonoscopy suggested possible inflammatory changes in the terminal ileum, ascending colon, hepatic flexure, and sigmoid colon. Based on subsequent hematoxylin-eosin (HE) staining and immunohistochemical analyses, the patient was diagnosed with intramucosal poorly differentiated invasive adenocarcinoma. The final diagnosis was Poorly Cohesive Carcinoma (PCC) with mucosal surface implantation in the whole gastrointestinal tract.</p> Conclusions <p>PCC typically corresponds to a poorly differentiated adenocarcinoma characterised by a high degree of malignancy. The occurrence of tumours of the same histological type diffusely involving the entire gastrointestinal tract is exceedingly rare.</p>