Tetralogy of Fallot is the most common cyanotic congenital heart disease. For decades, our institution has cared for humanitarian patients with late presentation of tetralogy of Fallot. They are characterized by severe right ventricular hypertrophy with consecutive diastolic dysfunction, increasing the risk of postoperative low cardiac output syndrome (LCOS). By right ventricular restrictive physiology, we hypothesized that patients receiving early postoperative beta-blockers (within 48 h after cardiopulmonary bypass) may have better diastolic function and cardiac output. This is a retrospective cohort study in a single-center tertiary pediatric intensive care unit. We included > 1-year-old humanitarian patients with a confirmed diagnosis of tetralogy of Fallot undergoing a complete surgical repair between 2005 and 2019. We measured demographic data, preoperative echocardiographic and cardiac catheterization measures, postoperative mean heart rate, vasoactive-inotropic scores, LCOS scores, length of stay, and mechanical ventilation duration. One hundred sixty-five patients met the inclusion criteria. Fifty-nine patients (36%) received early postoperative beta-blockers, associated with a lower mean heart rate, higher vasoactive-inotropic scores, and lower LCOS scores during the first 48 h following cardiopulmonary bypass. There was no significant difference in lengths of stay and ventilation. Conclusion: Early postoperative beta-blockers lower the prevalence of postoperative LCOS at the expense of a higher need for vasoactive drugs without any consequence on length of stay and ventilation duration. This approach may benefit the specific population of children undergoing a late complete repair of tetralogy of Fallot. What is Known: • Prevalence of low cardiac output syndrome is high following a late complete surgical repair of tetralogy of Fallot. What is New: • Early postoperative beta-blockade is associated with lower heart rate, prolonged relaxation time, and lower prevalence of low cardiac output syndrome. • Negative chronotropic agents like beta-blockers may benefit selected patients undergoing a late complete repair of tetralogy of Fallot, who are numerous in low-income countries.