This study evaluated the prognostic implications of post-percutaneous coronary intervention (PCI) neutrophil-to-lymphocyte ratio (NLR) in patients with acute myocardial infarction (AMI). A total of 309 patients with AMI who underwent cardiac magnetic resonance imaging (CMR) and a complete blood cell count within 24 hours before and after PCI were enrolled. Primary outcome was infarct size. Patients were assigned to high (n = 118) or low (n = 191) NLR groups according to the best cut-off value of 3.88. Infarct size (% of total left ventricular mass) was significantly higher in the high NLR group than in the low NLR group (24.1 ± 11.0 vs. 16.7 ± 9.1, p < 0.001). Post-PCI NLR ≥ 3.88 was associated with risk of a large-sized infarction (≥20%) (OR 2.91, 95% CI 1.73-4.88, p < 0.001). The risk of MACE was also significantly higher in the high NLR group than in the low NLR group (15.8% vs. 7.4%, HR 2.60, 95% CI 1.21-5.60, p = 0.015). Among patients with AMI who underwent PCI, high post-PCI NLR value was associated with higher risk of large-sized infarction as measured by CMR, as well as adverse clinical outcomes. Our findings suggest that post-PCI NLR is a useful tool for risk assessment in patients with AMI who undergo PCI.