Background: Iron deficiency (ID) is a prevalent condition in patients with cardiovascular diseases, irrespective of anemia, associated with adverse outcomes. Its incidence and prognostic value in acute coronary syndromes (ACS) are yet to be established. Current literature on the matter is scarce, and further research is necessary to confirm a clear link between ID and possible adverse outcome prediction in this group. Aims: This study aimed to evaluate the incidence and prognostic value of ID in ACS patients, and associations between iron parameters and patients’ characteristics, comorbidities, hospitalization length, laboratory results, electrocardiographic, echocardiographic assessment, and invasive coronary angiography results. Methods: We conducted an observational prospective study enrolling 214 consecutive patients after ACS. Adverse events were defined as all-cause death or non-elective rehospitalization due to cardiovascular causes. Results: ID patients constituted 46.7% of the studied cohort. ID was associated with higher NT-proBNP on admission (p = 0.03). Higher TSAT was independently associated with lower peak troponin levels (β = −0.03, standardized β = −0.15, p = 0.03). Ferritin < 100 ng/mL was paradoxically associated with shorter in-hospital stay (p = 0.03). In multivariable analysis, ID was an independent predictor of the composite endpoint (HR 1.94 [95% CI: 1.02–3.67], p = 0.04); however, no significant differences in event-free survival have been identified between ID and non-ID groups. Conclusions: ID is a common condition in ACS patients, associated with higher values of biomarkers reflecting cardiac damage, and may constitute an important predictor of adverse events after discharge. Further, larger, preferably multicenter studies are required to establish the exact association between ID and mortality among ACS patients treated with percutaneous coronary intervention.