Background: We aimed to investigate the association between electrocardiographic (ECG) grade III ischemia and angiographic thrombus burden in patients with acute ST-segment elevation myocardial infarction (STEMI) who underwent primary percutaneous coronary intervention (pPCI). Methods: The study population consisted of 307 patients with STEMI. Baseline ECGs of the patients were analyzed for grade III ischemia; angiographic thrombus burden was assessed by thrombolysis in myocardial infarction thrombus classification. Results: A total of 108 (35%) patients had low thrombus burden whereas 199 (65%) patients had high thrombus burden. Grade III ischemia was more prevalent in patients with high thrombus burden (25.1% vs 11.1%, P = .004). Only grade III ischemia (odds ratio: 2.59, 95% confidence interval 1.24-5.39, P = .011) and history of coronary artery disease (CAD) were found to be the independent predictors of high thrombus burden. Conclusion: Grade III ischemia on ECG and previous history of CAD were independent predictors of coronary thrombus burden in patients with STEMI who underwent pPCI.