Myocardial ischemia/reperfusion (I/R) injury is a clinically fatal disease, caused by restoring myocardial blood supply after a period of ischemia or hypoxia. However, the underlying mechanism remains unclear. Recently, increasing evidence reveal that microRNAs (miRs) participate in myocardial I/R injury. This study aimed to investigate whether miR-128-1-5p contributed to cardiomyocyte apoptosis induced by myocardial I/R injury. Here, we showed that the expression of miR-128-1-5p was decreased in mice following myocardial I/R injury. Down-regulation of miR-128-1-5p was also showed in H9c2 cardiomyocytes after hypoxia/reoxygenation (H/R), and in neonatal rat cardiomyocytes (NRCMs) with H2O2 treatment. Importantly, we found that overexpression of miR-128-1-5p ameliorates cardiomyocyte apoptosis both in H9c2 cardiomyocytes and NRCMs. Moreover, we also found that growth arrest DNA damage-inducible gene 45 gamma (Gadd45g) is identified as a direct target of miR-128-1-5p, which negatively regulated Gadd45g expression. Additionally, silencing of Gadd45g inhibits cardiomyocyte apoptosis in H9c2 cardiomyocytes and NRCMs. These results reveal a novel mechanism by which miR-128-1-5p regulates Gadd45g-mediated cardiomyocyte apoptosis in myocardial I/R injury.