A leftward motion of the ventricular septum prior to ejection, known as the septal flash (SF), is frequently observed in patients with left bundle-branch block (LBBB). We investigated whether the abnormal motion of the ventricular septum affects right ventricle (RV) contractile performance in LBBB patients with preserved left ventricular ejection fraction (LVEF). Forty-four patients with complete LBBB were selected using standard 12-lead electrocardiograms (ECGs), with 30 healthy individuals serving as controls. According to the presence of SF, patients with LBBB were allocated to two subgroups: those with SF (LBBB-SF, n = 24) and those without SF (LBBB-NSF, n = 20). RV longitudinal strain (LS) decreased in LBBB patients with preserved LVEF compared to control subjects (p = 0.002). And RV LS decreased significantly in LBBB-SF patients compared to NSF-LBBB patients (p = 0.04). RV LS correlated negatively with involved septal myocardial segments of SF (r = -0.36, p = 0.02), but did not correlate with the magnitude of SF. RV contractile performance deceased in LBBB patients with preserved LVEF. SF, particularly the extent of this phenomenon, may further affect RV contractile performance.