Background Heterozygous familial hypercholesterolemia (HeFH) more likely exhibits extensive atherosclerotic disease at multiple vascular beds. Lipoprotein(a) (Lp(a)) is an atherogenic lipoprotein that elevates HeFH-related atherosclerotic cardiovascular disease risks. Whether circulating Lp(a) level associates with polyvascular propagation of atherosclerosis in subjects with HeFH remains uncertain. Methods and Results The current study analyzed 370 subjects with clinically diagnosed HeFH who received evaluation of systemic arteries. Polyvascular disease (polyVD) was defined as more than 2 coexisting atherosclerosis conditions including coronary artery disease, carotid stenosis, or peripheral artery disease. Clinical characteristics and lipid features were analyzed in subjects with HeFH and polyVD; 5.7% of patients with HeFH (21/370) had polyVD. They were more likely to have a clustering of risk factors, tendon (<0.001) and skin xanthomas (=0.004), and corneal arcus (=0.026). Furthermore, an elevated Lp(a) level (=0.006) and a greater frequency of Lp(a) level ≥50 mg/dL (<0.001) were observed in subjects with HeFH and polyVD. On multivariable analysis adjusting risk factors and lipid-lowering agents, Lp(a) ≥50 mg/dL (odds ratio [OR], 5.66 [95% CI, 1.68-19.0], =0.005), age, and family history of premature coronary artery disease independently predicted polyVD in subjects with HeFH. Of note, the prevalence of polyVD rose to 33.3% in patients with HeFH and age >58 years old, family history of premature coronary artery disease, and Lp(a) ≥50 mg/dL (OR, 10.3 [95% CI, 3.12-33.4], <0.001). Conclusions An increased level of circulating Lp(a) levels predicted concomitance of polyVD in patients with HeFH. The current findings suggest subjects with HeFH and Lp(a) ≥50 mg/dL as a high-risk category who require meticulous screening of systemic vascular beds.