Mitochondrial acyl-CoA dehydrogenase 9 (ACAD9) deficiency is one of the common causes of respiratory chain complex I deficiency, which is characterized by cardiomyopathy, lactic acidemia, and muscle weakness. Infantile cardiomyopathy is the most common phenotype and is usually lethal by the age of 5 years. Riboflavin treatment is known to be effective in ~65% of the patients; however, the remaining are unresponsive to riboflavin and are in need of additional treatment measures. In this report, we describe a patient with ACAD9 deficiency who developed progressive cardiomyopathy at 8 months of age. As the patient’s left ventricular ejection fraction (LVEF) kept decreasing to 45.4% at 1 year 8 months, sodium pyruvate treatment was introduced together with a beta-blocker and coenzyme Q10. This resulted in a steady improvement, with full and sustained normalization of cardiac function without riboflavin. The therapy, therefore, might be a useful addition for the treatment of ACAD9 deficiency.