Background: Prediabetes (preT2D) is considered a subtle adverse cardiovascular (CV) risk factor after acute myocardial infarction. Glycated hemoglobin (HbA1c) ranges to identify preT2D are different between ADA and WHO guidelines (5.7–6.4 vs. 6.0–6.4%, respectively). Aim: To evaluate the prognostic value of HbA1c different preT2D-ranges and their correlation with demographic, instrumental, and laboratory parameters in STEMI. Methods: A total of 1681 patients (mean age 67 ± 13 years; 1217 males) were enrolled. Admission HbA1c was used to identify patients with no-T2D (<5.7%), HbA1c 5.70–5.99%, and WHO-preT2D with HbA1c 6–6.49%, and T2D (HbA1c ≥ 6.5). Results: HbA1c 5.7–5.99, WHO-preT2D, and T2D progressively correlated with an increasing number of CV risk factors. However, only T2D, but not preT2D, was significantly associated with adverse prognosis (in-hospital and one-year death). Conclusions: PreT2D is correlated with CV risk factors, but not with adverse prognosis as compared to no-T2D. Nonetheless, routine HbA1c testing in the STEMI population and HbA1c-5.7–5.99 patient inclusion in the preT2D category may help to identify those who may benefit from intervention and lifestyle strategies to early prevent preT2D progression.