Objectives: We aimed to investigate the association between the triglyceride glucose index (TGI) and mortality in patients with ST-elevation myocardial infarction (STEMI). Methods: This retrospective study utilized data from the records of patients diagnosed with STEMI who underwent primary percutaneous coronary intervention (PCI) at the Cardiology Department of Private Aktif International Hospital between 2020 and 2023. Demographic data, medical history, laboratory results, and treatment processes of the patients were obtained from retrospective records. Patients were divided into low (TGI ≤ 8.6)-, medium (TGI = 8.6–9.2)-, and high (TGI ≥ 9.2)-TGI groups according to their TGI levels. Results: The average age of the patients was 62 ± 10 years, and 65% were men. The intracoronary thrombus burden of patients in the high-TGI group was found to be significantly higher compared to the low- and medium-TGI groups (p = 0.01). While the rate of patients with a thrombolysis in myocardial infarction (TIMI) thrombosis score of 3 or above was 45% in the high-TGI group, this rate was observed to be 20% in the low-TGI group. The short-term (30-day) mortality rate was found to be 15% in the high-TGI group, 8% in the medium-TGI group, and 5% in the low-TGI group (p = 0.02). Long-term (after 30 days) mortality rates were determined as 25% in the high-TGI group, 15% in the medium-TGI group, and 10% in the low-TGI group (p = 0.01). TGI was an independent risk factor for both short-term and long-term mortality. The hazard ratio (HR) of high TGI levels for short-term mortality was found to be 2.5 (95% CI: 1.5–4.1, p = 0.01), and the HR for long-term mortality was 2.0 (95% CI: 1.3–3.2, p < 0.02). Conclusions: Our results show that high TGI levels are associated with increased thrombus burden and high mortality rates in STEMI. TGI can be used not only in predicting STEMI but also in early risk stratification and treatment planning for STEMI patients.