The adult mammalian heart has limited regenerative capacity due to the low proliferative ability of cardiomyocytes, whereas embryonic cardiomyocytes exhibit robust proliferative potential. Using single-cell RNA sequencing of embryonic hearts, we identified prothymosin α (PTMA) as a key factor driving cardiomyocyte proliferation. Overexpression of PTMA in primary mouse and rat cardiomyocytes significantly promoted cardiomyocyte proliferation and similarly enhanced proliferation in human iPSC-derived cardiomyocytes. Conditional knockout of Ptma in cardiomyocytes impaired neonatal heart regeneration. AAV9-mediated overexpression of Ptma extended the neonatal proliferative window and showed therapeutic promise for enhancing adult heart regeneration. Mechanistically, PTMA interacted with MBD3, inhibiting its deacetylation activity within the MBD3/HDAC1 NuRD complex. This inhibition increased STAT3 acetylation, which positively regulated STAT3 phosphorylation and activation of its target genes. These findings establish PTMA as a critical regulator of heart regeneration and suggest its potential as a therapeutic target for ischemic myocardial injury.