Barth syndrome (BTHS) is a rare disease caused by mutations in the tafazzin gene that affects the heart and muscles; however, to date, no clinically effective drugs are available. In BTHS, mitochondrial function is reduced owing to changes in cardiolipin metabolism. We developed mitochonic acid 5 (MA-5), a small-molecule compound that increases ATP levels, improves mitochondrial dynamics, and is effective in treating mitochondrial and muscle diseases. Therefore, this study examined the effectiveness of MA-5 in treating BTHS. The mitochondrial functions of four isolated BTHS skin fibroblasts were examined. Human BTHS induced pluripotent stem cell (iPSC) were differentiated into myoblasts and cardiolipin metabolism and mitochondrial functions were analyzed. RNA-seq was performed to clarify the metabolic changes. Using a Drosophila melanogaster model of BTHS, the effects of MA-5 on motor performance and cardiac phenotype were examined. MA-5 improved mitochondrial function and reduced cell death due to oxidative stress in skin fibroblasts of patients with BTHS. MA-5 promoted ATP production and reduced oxidative stress in human BTHS iPS cell-derived myoblasts. RNA-seq analysis revealed that MA-5 alleviated endoplasmic reticulum stress in BTHS cells. Administration of MA-5 to BTHS Drosophila improved locomotor ability and tachycardia observed in patients with BTHS. Protein interaction analyses suggested colocalization of ATPase and the MA-5-binding protein mitofilin. These data suggested that MA-5 improves BTHS dysfunction and may serve as a novel therapeutic agent for BTHS.