Early initiation of reperfusion therapy remains the cornerstone of successful management for ST-elevation myocardial infarction (STEMI). Rapid restoration of coronary blood flow relies on prompt recognition of the typical ST-segment elevation on a 12-lead electrocardiogram (ECG)—a surrogate for coronary occlusion or critical stenosis—allowing timely activation of the STEMI protocol cascade, with a major positive impact in mortality and clinical outcomes. However, atypical, very high risk ECG patterns—known as “STEMI equivalents”—are present in 10% to 25% of patients with ongoing myocardial ischemia in need of urgent primary percutaneous coronary intervention. Though briefly mentioned in the current recommendations, structured clinical data on those specific ECG presentations are lacking. By thoroughly searching MEDLINE and EMBASE we conducted a structured review of non-STEMI, albeit very high risk, ECG patterns of acute coronary syndrome, often associated with coronary occlusion or critical stenosis. After screening 997 studies, we identified the following distinct “STEMI equivalent” ECG patterns: Wellens’ syndrome, de Winter sign, hyperacute T waves, left bundle branch block—including paced rhythm—and right bundle branch block. For each pattern, a brief summary of the existing evidence, together with the sensitivity, specificity, and positive predictive value—whenever available—are presented. In conclusion, prompt recognition of “STEMI equivalent” ECG patterns is crucial for every physician or paramedic dealing with acute coronary syndrome patients in the emergency department or the prehospital setting, as misinterpretation of those high risk presentations can lead to reperfusion delays and worse outcomes.