This study demonstrates the in vivo reprogramming of cardiac fibroblasts into cardiomyocyte-like cells (iCMs) in the murine heart following local delivery of the transcription factors Gata4, Mef2c, and Tbx5 (GMT). The authors used genetic lineage-tracing to show that resident non-myocytes in the heart can be reprogrammed into iCMs after coronary ligation. These iCMs exhibited bi-nucleate morphology, assembled sarcomeres, and expressed cardiomyocyte-like genes. Single-cell analysis revealed ventricular iCMs with action potentials, beating upon electrical stimulation, and evidence of electrical coupling. In vivo delivery of GMT reduced infarct size and improved cardiac function up to 3 months post-injury. Co-delivery with Thymosin β4 further enhanced scar area reduction and cardiac function. These findings suggest that cardiac fibroblasts can be reprogrammed into cardiomyocyte-like cells in their native environment, offering potential for regenerative therapy.This study demonstrates the in vivo reprogramming of cardiac fibroblasts into cardiomyocyte-like cells (iCMs) in the murine heart following local delivery of the transcription factors Gata4, Mef2c, and Tbx5 (GMT). The authors used genetic lineage-tracing to show that resident non-myocytes in the heart can be reprogrammed into iCMs after coronary ligation. These iCMs exhibited bi-nucleate morphology, assembled sarcomeres, and expressed cardiomyocyte-like genes. Single-cell analysis revealed ventricular iCMs with action potentials, beating upon electrical stimulation, and evidence of electrical coupling. In vivo delivery of GMT reduced infarct size and improved cardiac function up to 3 months post-injury. Co-delivery with Thymosin β4 further enhanced scar area reduction and cardiac function. These findings suggest that cardiac fibroblasts can be reprogrammed into cardiomyocyte-like cells in their native environment, offering potential for regenerative therapy.