Trimetazidine (TMZ) improves left ventricular (LV) function and alleviates angina. TMZ is a metabolism-related drug, but its molecular actions and non-metabolic effects have not yet been elucidated. In this study, we investigated whether TMZ improves right ventricular (RV) function and decreases apoptosis in RV myocardial cells (RVMCs) by regulating miRNA-21 (miR-21) expression in vitro and in vivo. We used cultivated RVMCs and LV myocardial cells (LVMCs) and provided hypoxic stimulation for 24 h to induce MC apoptosis. RVMCs showed more severe apoptosis as indicated by terminal deoxynucleotidyl-transferase-mediated dUTP nick end-labeling (TUNEL) staining and caspase-3 protein and activity assays. The decrease in miR-21 expression was more dramatic in RVMCs. Subsequently, TMZ (10 µM) was added to the RVMCs prior to hypoxic stimulation. The TMZ-treated RVMCs showed less apoptosis and an increased expression of miR-21. The transfection of RVMCs with a miR-21-specific inhibitor weakened the protective effects of TMZ. To evaluate TMZ effectiveness in right heart failure, we used a combination treatment of hypoxia and the vascular endothelial growth factor receptor blocker, Su5416, to construct a stable model, and administered TMZ. TMZ improved RV function (as indicated by an increase in tricuspid annular plane systolic excursion), and inhibited fibrosis. TMZ also protects RVMCs againts apoptosis and increases miR-21 expression.