Propionic acidemia is a congenital metabolic disorder that typically develops in the neonatal period, with a metabolic crisis comprising metabolic acidosis and hyperammonemia. However, late-onset cardiomyopathy is an initial presentation in a limited number of patients. The present case was a 14-year-old female with mental retardation and epilepsy. During a cardiac screening test conducted in school, tachycardia was observed, and a diagnosis of dilated cardiomyopathy was reached. The acylcarnitine analysis showed a high level of propionylcarnitine; thus, propionic acidemia was strongly suspected. Genetic analysis identified compound heterozygous mutations in the PCCB gene. Myocardial biopsy showed hypertrophy of cardiomyocytes with vacuolar changes under light microscopy, and the number of mitochondria was increased under electron microscopy. When differentiating cardiomyopathy in children and adolescents, physicians should be aware of the possibility of inborn errors of metabolism as a cause.