BACKGROUND Triangular QRS-ST-T waveform (TW) electrocardiography pattern has been found to be associated with poor prognosis in patients with ST-segment elevation myocardial infarction (STEMI). It identifies a subset of patients at high risk of both ventricular fibrillation and cardiogenic shock, with high in-hospital mortality. Therefore, aggressive treatment is needed in patients presenting with this electrocardiography pattern. However, this pattern is rarely present in non-ischemic cardiac diseases. CASE REPORT We report the case of a 50-year-old man who came to our emergency room with a chief complaint of gastrointestinal problems and partial bowel obstruction. After failure of initial conservative treatment, laparotomy was planned. Just before the surgery, the patient felt a non-specific chest discomfort and showed ST-segment elevation on ECG and slight elevation of cardiac enzyme. He was then treated for STEMI with an intravenous thrombolytic. However, the degree of ST-segment elevation further increased and showed a TW pattern. Transthoracic echocardiography revealed a moderate pericardial effusion with normal ejection fraction and a normokinetic left ventricle; hence, a diagnosis of acute myopericarditis was made. After treatment with low-dose steroid and colchicine, his symptoms improved, the electrocardiography pattern returned to normal, and the gastrointestinal symptoms resolved. CONCLUSIONS To the best of our knowledge, this is the first case report of an acute myopericarditis presenting with a TW electrocardiography pattern. Myopericarditis should always be considered in the differential diagnosis of acute chest pain and ST segment electrocardiography changes, including TW pattern. The use of echocardiography can help determine the diagnosis of myopericarditis.