AIMS: Right ventricular (RV) dysfunction is a major determinant of outcomes after Tetralogy of Fallot repair (rToF). Although right bundle branch block is present in most patients and electro-mechanical dyssynchrony (EMD) has been associated with RV dysfunction, only a subset of patients ultimately develops RV dysfunction. We hypothesized that septal dysfunction is a key feature associated with the development of EMD-induced RV dysfunction in rToF. METHODS: We retrospectively studied 126 pediatric rToF patients (median age 14.7 years) with contemporaneous ECG, echocardiography, and cardiac magnetic resonance (CMR) between 2019-2025. Septal mechanics were quantified by strain-derived systolic rebound stretch (SRS) and further combined with RV free-wall mechanics into a systolic stretch index (SSI). RV mechanical dyssynchrony was visually classified, with septal contraction abrogation and systolic stretch as key features. Multivariable models evaluated associations between systolic stretch parameters and RV dysfunction (CMR-derived RV ejection fraction (EF) ≤ 46%), adjusting for relevant covariates. RESULTS: RV dysfunction was present in 24.6% of patients. Patients with >moderate RV outflow tract obstruction (peak gradient >40 mmHg) were excluded, median pulmonary regurgitant fraction was 35% (IQR, 26-44%). Median SSI was higher in patients with RV dysfunction than in those without (8.9% (IQR 3.2-11.3) vs 2.9% (IQR 1.5-5.5), p<0.001). Premature termination of septal deformation with subsequent stretching, quantified by SRS, was associated with RV dysfunction (p<0.001). Worse septal stretch patterns were associated with RV dysfunction, larger RV end-systolic indexed volume, and longer QRSd (p<0.001). Higher SSI was independently associated with lower RVEF (p<0.001) after adjustment for age, QRSd, RV dilation, and surgical substrate (p<0.001), whereas QRSd lost significance on RVEF after adjusting for SSI. RVEF was lower at SSI values >4%. SSI demonstrated excellent intraobserver and interobserver reproducibility (ICC 0.996 and 0.997, respectively). CONCLUSIONS: In pediatric rToF, RV dysfunction was associated with abnormal septal contraction and systolic stretch. RV-SSI, integrating inefficient RV mechanics via septal and RV free-wall systolic stretch, was independently associated with RV dysfunction beyond QRSd, RV volumes and pulmonary regurgitation severity. These findings may help characterize RV electromechanical phenotypes and provide a rationale for future studies evaluating RV resynchronization therapy.