Dystrophin deficiency leads to lethal dilated Duchenne cardiomyopathy. A promising therapy is to deliver a highly abbreviated microdystrophin gene to the heart using adeno-associated virus (AAV). Microdystrophin has been shown to mitigate dystrophin-deficient skeletal muscle disease. However, it is not clear whether microdystrophin is equally effective in treating Duchenne cardiomyopathy. To evaluate microdystrophin therapy in the heart, we injected 5 × 10¹² viral genome particles/mouse of AAV-9 ΔR4-23/ΔC microdystrophin vector via tail vein to ~16–20-month-old (average 18.7–month-old) female mdx mice, a manifesting model of Duchenne cardiomyopathy. Cardiac transduction and heart function were examined at 2–8 months after gene transfer. We observed robust myocardial microdystrophin expression. Electrocardiography (ECG) and left ventricular catheter hemodynamic assays also revealed significant improvement. Furthermore, AAV-microdystrophin therapy prevented dobutamine-stress induced acute cardiac death. We demonstrate for the first time that AAV microdystrophin therapy significantly ameliorates functional deficiency in a phenotypic model of Duchenne cardiomyopathy. Our results support further exploration of microdystrophin therapy to treat Duchenne cardiomyopathy.