The formation of extracellular traps (ETs) through ETosis has emerged as a key mechanism in immunothrombosis. However, the temporal dynamics and clinical significance of ETosis in coronary thrombi of ST-elevation myocardial infarction (STEMI) patients remain incompletely understood. We investigated whether ETosis burden increases with thrombus age and is associated with DNASE1 and TREX1 genetic variants as well as impaired myocardial reperfusion. Thrombus aspirates from 81 STEMI patients undergoing primary percutaneous coronary intervention were histologically classified as fresh (n = 41) or lytic (n = 40). ETosis was quantified by citrullinated histone H3 (CitH3) immunohistochemistry and digital image analysis, complemented by multiplex staining for myeloperoxidase (MPO), CD68, caspase 3, and CD61. Plasma ET-related markers and genotyping of DNASE1 (rs1053874) and TREX1 (rs11797) were also performed. CitH3-positive cells were present in all thrombi but were more abundant in lytic (older) thrombi compared with fresh thrombi (1348 vs. 591 cells/mm2, p < 0.001). Increased ETosis was associated with neutrophil and macrophage infiltration, apoptosis, prolonged ischemia time, elevated systemic inflammation (neutrophil-lymphocyte ratio and C-reactive protein), and impaired myocardial reperfusion (lower TIMI flow grades). Moreover, the DNASE1 GG genotype was associated with higher densities of MPO- and CD68-positive cells, whereas the TREX1 CC genotype was associated with increased densities of CitH3-, MPO-, and CD68-positive cells. This study demonstrates that ETosis increases with coronary thrombus maturation and is associated with local inflammation and impaired reperfusion in STEMI. Genetic variants in DNASE1 and TREX1 may modulate inflammatory cell accumulation within thrombi. These findings suggest ETosis as a potential therapeutic target, particularly in patients with delayed presentation.