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Introduction Reduced exercise performance is common in patients with repaired tetralogy of Fallot (rToF) and relates to adverse outcomes. The effects of high-intensity interval training (HIIT) in rToF are unknown. Furthermore, it is unknown if home-based, online-monitored exercise training is effective in rToF. We aimed to examine the feasibility, safety and effects of home-based HIIT in patients with rToF.Methods Patients aged 12–30 years (n=34, median age 16 (IQR 14–23)) with rToF were randomised to a 12-week home-based HIIT programme or standard of care (SOC). HIIT was prescribed in three 30 min sessions/week, with heart rate telemonitoring. SOC included exercise as usual. Co-primary end points were mean differences in peak oxygen consumption (VO2) and maximal workload, analysed using linear mixed-effects (LME) models. Secondary end points were: echocardiography, ECG and MRI-derived measures of cardiac structure and function.Results No serious adverse events occurred. After HIIT, peak VO2 increased from 38.3 (34.0–42.5) mL/kg/min to 40.9 (36.6–45.2) mL/kg/min (∆ 4.0 mL/kg/min, compared with control period (95% CI 1.52 to 6.53)), workload increased from 204 (174–234) W to 218 (188–248) W (∆ 17 W, compared with control period (95% CI 5 to 30)). No changes in cardiac volumes and ejection fraction were observed after HIIT. VO2 improved after HIIT independent of baseline fitness, body mass index and biventricular size. Notably, lower right ventricular ejection fraction (RV EF) was associated with greater improvement.Conclusion HIIT significantly enhances exercise capacity in adolescents and young adults with rToF without adverse effects on cardiac structure or function, although the sample size limits the detection of subtle changes and can be successfully performed at home with online monitoring. This intervention offers a feasible alternative to traditional, on-site supervised exercise programmes, for patients with rToF, including those with lower RV EF.Trial registration number NTR2731.