IMPORTANCE: Many pediatric patients with heterozygous familial hypercholesterolemia (HeFH) cannot reach recommended low-density lipoprotein cholesterol (LDL-C) concentrations on statins alone and require adjunct lipid-lowering therapy (LLT); the use of alirocumab in pediatric patients requires evaluation. OBJECTIVE: To assess the efficacy of alirocumab in pediatric patients with inadequately controlled HeFH. DESIGN, SETTING, AND PARTICIPANTS: This was a phase 3, randomized clinical trial conducted between May 2018 and August 2022 at 43 centers in 24 countries. Pediatric patients aged 8 to 17 years with HeFH, LDL-C 130 mg/dL or greater, and receiving statins or other LLTs were included. Following consecutive enrollment into dosing cohorts, 25 of 99 patients screened for dosing every 2 weeks (Q2W) failed screening; 25 of 104 patients screened for dosing every 4 weeks (Q4W) failed screening. A total of 70 of 74 Q2W patients (95%) and 75 of 79 Q4W patients (95%) completed the double-blind period. INTERVENTIONS: Patients were randomized 2:1 to subcutaneous alirocumab or placebo and Q2W or Q4W. Dosage was based on weight (40 mg for Q2W or 150 mg for Q4W if <50 kg; 75 mg for Q2W or 300 mg for Q4W if ≥50 kg) and adjusted at week 12 if LDL-C was 110 mg/dL or greater at week 8. After the 24-week double-blind period, patients could receive alirocumab in an 80-week open-label period. MAIN OUTCOMES AND MEASURES: The primary end point was percent change in LDL-C from baseline to week 24 in each cohort. RESULTS: Among 153 patients randomized to receive alirocumab or placebo (mean [range] age, 12.9 [8-17] years; 87 [56.9%] female), alirocumab showed statistically significant reductions in LDL-C vs placebo in both cohorts at week 24. Least squares mean difference in percentage change from baseline was -43.3% (97.5% CI, -56.0 to -30.7; P < .001) Q2W and -33.8% (97.5% CI, -46.4 to -21.2; P < .001) Q4W. Hierarchical analysis of secondary efficacy end points demonstrated significant improvements in other lipid parameters at weeks 12 and 24 with alirocumab. Two patients receiving alirocumab Q4W experienced adverse events leading to discontinuation. No significant difference in adverse event incidence was observed between treatment groups. Open-label period findings were consistent with the double-blind period. CONCLUSIONS AND RELEVANCE: The findings in this study indicate that alirocumab Q2W or Q4W significantly may be useful for reducing LDL-C and other lipid parameters and be well tolerated in pediatric patients with HeFH inadequately controlled with statins. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT03510884.