BACKGROUND The clinical significance of QT prolongation in pediatric hypertrophic cardiomyopathy is unclear. OBJECTIVE This study aimed to determine the prevalence, risk factors, and outcomes of QT prolongation in pediatric patients with hypertrophic cardiomyopathy. METHODS Phenotype-positive, pediatric patients with primary hypertrophic cardiomyopathy (P-HCM) (n = 212) and RASopathy hypertrophic cardiomyopathy (n = 55) were included. Corrected JT (JTc) interval as a measure of QT prolongation was calculated at baseline and last follow-up. Factors associated with JTc duration were analyzed using a generalized estimating equation model. Association of JTc prolongation (JTc of > 370 ms) with risk of sudden cardiac death (SCD) events was analyzed. SCD events were defined as a composite of SCD, resuscitated SCD event, or appropriate shock from a primary prevention implantable cardioverter-defibrillator. RESULTS Notably, 24% of patients with P-HCM and 44% of patients with RASopathy hypertrophic cardiomyopathy had prolonged JTc (P = .004). JTc had only a modest correlation with the severity of left ventricular hypertrophy. In P-HCM, JTc prolongation was associated with SCD events on multivariable analysis (hazard ratio 2.9 [1.2-6.8], P = .016); 5-year SCD event-free survival from baseline evaluation was 86% in P-HCM. Including JTc as a risk factor improved the c-statistic for 5-year SCD risk prediction to 0.85 compared with 0.73 when using Precision Medicine in Cardiomyopathy risk scores alone and to 0.73 compared from 0.70 when using hypertrophic cardiomyopathy Risk-Kids scores alone. CONCLUSION JTc prolongation was independently associated with the risk of SCD events. Including JTc prolongation with SCD risk scores improved 5-year SCD risk prediction for P-HCM. This has implications for closer SCD risk monitoring in patients with P-HCM with JTc prolongation.