We report 3 cases of gastric cancer with rare presentations of oncologic emergency. Case 1 involved cardiac tamponade caused by mediastinitis-induced pericarditis. Metastatic lymph nodes became enlarged after total gastrectomy and invaded the esophagojejunal anastomotic region. Then, a fistula to the mediastinum occurred, resulting in mediastinitis. The drainage was efficacious, but the patient died of cancer 1 month after admission. Case 2 involved cardiac tamponade caused by invasion of metastatic peritoneal tumor into the pericardium. The drainage was successful, but the patient died of cancer 2 weeks after drainage. Case 3 involved perforation of the stomach during the third course of chemotherapy. A total gastrectomy was urgently performed. Thereafter, chemotherapy was continued. However, the patient died of cancer 6 months later. These oncologic emergencies should be considered, although they are rare.