Objective To explore the role and underlying mechanisms of miR-433-3p in hydrogen peroxide (H₂O₂ )-induced injury to H9c2 cardiomyocytes. Methods An oxidative stress injury model of H9c2 cardiomyocytes was established. H9c2 cardiomyocytes were transfected with miR-433-3p mimics, miRNA mimic negative control (miR-NC), pcDNA-NC, and a MAPK8 overexpression plasmid (pcDNA-MAPK8) and treated with H₂O₂ . H9c2 cells were divided into Control, H₂O₂ , H₂O₂+ miR-NC, H₂O₂+ miR-433-3p mimic, H₂O₂+ miR-433-3p mimic + pcDNA-NC, and H₂O₂+ miR- 433-3p mimic + pcDNA-MAPK8 groups. The mRNA expressions of miR-433-3p and MAPK8 in H9c2 cells were detected by qRT-PCR assay. Cell viability and the amount of lactate dehydrogenase (LDH) released were detected by MTT assay and ELISA kits, respectively. Cell apoptotic-related protein expressions of Bax, Bcl-2, caspase-3, and cleaved caspase-3 were measured by Western blot analysis. The luciferase reporter assay was performed for testing the targeting relationship between miR-433-3p and MAPK8. Results Compared with the control group, the expression of miR-433-3p was lower in H₂O₂ -induced H9c2 cardiomyocytes. Compared with the H₂O₂ + miR-NC group, miR-433-3p overexpression significantly reduced the amount of LDH released and enhanced cell viability. In addition, compared with the H₂O₂ + miR-NC group, miR-433-3p overexpression significantly decreased the expressions of the pro-apoptotic proteins Bax and cleaved caspase-3, but increased the expression of the anti-apoptotic protein Bcl-2. The luciferase reporter assay showed that miR-433-3p directly targeted MAPK8 and negatively regulated the expression of MAPK8. Overexpression of MAPK8 reversed the inhibitory effects of miR-433-3p overexpression in H₂O₂ -induced H9c2 cell injury and apoptosis. Conclusions miR-433- 3p has a protective role in cardiomyocyte injury induced by H₂O₂ through the negative regulation of MAPK8.