Background The aim of this study was to investigate the association of serum lipoprotein(a) [Lp(a)] concentrations with the angiographic extent of coronary artery disease and selected angiographic findings in patients hospitalized with acute coronary syndromes (ACS). Methods This retrospective, single-center study included 1009 consecutive patients hospitalized with ACS, including unstable angina (UA), non-ST-segment elevation myocardial infarction (NSTEMI), and ST-segment elevation myocardial infarction (STEMI), between November 2022 and February 2025. Serum Lp(a) concentrations were measured routinely within the first 24 h of hospitalization. Associations between Lp(a) concentrations and angiographic findings were evaluated using multivariable logistic regression adjusted for major cardiovascular risk factors. Results The median Lp(a) concentration was 23.8 nmol/L, and elevated Lp(a) concentrations (≥105 nmol/L) were observed in 24.9% of patients. Higher Lp(a) concentrations were associated with a greater angiographic extent of coronary artery disease, including an increased prevalence of three-vessel disease and chronic total occlusions (CTO). In multivariable analyses, each 50 nmol/L increase in Lp(a) was associated with higher odds of three-vessel disease (OR 1.16, 95% CI 1.08–1.24; p < 0.001) and CTO (OR 1.17, 95% CI 1.06–1.29; p = 0.003). Using the clinically recommended threshold of ≥105 nmol/L, elevated Lp(a) remained independently associated with three-vessel disease (OR 1.51, 95% CI 1.12–2.03; p = 0.008) and CTO (OR 1.84, 95% CI 1.17–2.90; p = 0.009). Patients with premature STEMI exhibited the highest Lp(a) concentrations among all ACS subgroups. Among routinely measured lipid parameters, Lp(a) demonstrated the highest discriminatory performance for identifying CTO, although the overall discriminatory ability was modest. Conclusions Elevated Lp(a) concentrations were independently associated with the angiographic extent of coronary artery disease in patients with acute coronary syndromes, including multivessel disease and chronic total occlusions. These findings demonstrate a cross-sectional association between a single Lp(a) measurement obtained during the index hospitalization and selected angiographic findings. Further prospective studies are required to confirm these associations and determine their clinical significance.