Background: Localizing the site of origin of outflow tract ventricular arrhythmias (OTVA) before ablation remains challenging despite existing ECG algorithms. Anatomical alterations of the aortic root associated with left ventricular outflow tract (LVOT) origin can be visualized through aortoseptal angle measurement. This study investigated the aortoseptal angle for predicting LVOT origin and improving the predictive power of current ECG algorithms. Methods: Patients undergoing successful OTVA ablation at a national cardiovascular referral hospital were enrolled. Transthoracic echocardiography measured the aortoseptal angle before ablation. Patients with structural abnormalities, multifocal arrhythmias, reduced ejection fraction, unsuccessful ablation, or non-inducible VAs were excluded. Multivariable logistic regression was used to develop a diagnostic model. Results: Forty-eight patients were included (44.9 ± 12.2 years, 79.8% female). Patients with LVOT origin (17.1%) had higher hypertension prevalence (71.4% vs. 17.6%; p < 0.001) and smaller aortoseptal angles (128.0 ± 6.2 vs. 136.7 ± 5.8; p < 0.001) than right ventricular outflow tract (RVOT) origin. The aortoseptal angle demonstrated good diagnostic performance (AUC 85.5%), comparable to ECG algorithms (V2 transition ratio 90.3%, V2S/V3R index 81.4%). Multivariable analysis identified three independent predictors: hypertension (OR 10.35; 95% CI: 1.07–99.87; p = 0.043), aortoseptal angle (OR 0.79; 95% CI: 0.65–0.96; p = 0.033), and V2 transition ratio (OR 12.12; 95% CI: 1.03–141.48; p = 0.026). The integrated model substantially improved performance (AUC 0.966, sensitivity 92.31%, specificity 88.24%). Conclusions: Aortoseptal angle measurement by echocardiography effectively identifies LVOT origin. Integration with hypertension status and V2 transition ratio substantially enhances diagnostic accuracy, supporting the use in pre-ablation assessment. © 2026 The Author(s). Journal of Arrhythmia published by John Wiley & Sons Australia, Ltd on behalf of Japanese Heart Rhythm Society.