Objective Acute myocardial infarction (AMI) is a highly prevalent and deadly disease globally, with its incidence continuing to rise in recent years. Timely reperfusion therapy is crucial for improving the prognosis of AMI patients. However, myocardial reperfusion can lead to irreversible myocardial ischemia/reperfusion (MI/R) injury, which is associated with adverse cardiovascular outcomes following AMI. Studies have shown that microRNAs (miRNAs) are abnormally expressed during MI/R injury and play an important role in the fate of cardiomyocytes. Effective preventive and therapeutic strategies against MI/R injury remain lacking in clinical practice, necessitating elucidation of the molecular mechanisms underlying MI/R onset and progression. This study investigated the role of microRNA-878 (miR-878) in the regulation of mitochondria-mediated apoptosis in MI/R injury.Methods The H9c2 cells were flushed with a gas mixture containing 1% O₂, 5% CO₂ and 94% N₂ for 3 h. Then the cells were incubated in complete culture medium under 5% CO₂ and 95% air for 6 h to mimic in vivo hypoxia/reoxygenation (H/R) injury. Cell viability were detected by CCK-8 assay. The concentrations of lactate dehydrogenase (LDH) were then measured.The level of apoptosis was analyzed by flow cytometry. The morphology of mitochondria was analyzed by immunofluorescence and laser confocal microscopy. The levels of mitochondrial reactive oxygen species (mtROS) were detected by immunofluorescence. Dual luciferase reporter gene assay was used to study the binding site of miR-878 and Pim1. RNA immunoprecipitation (RIP) assay was used to verify the binding relationship between miR-878 and Pim1. The gene expression levels were detected by real-time fluorescent quantitative PCR (RT-qPCR) and Western blot.Results The study found that compared with the control group, the expression of miR-878 in H/R-treated H9c2 cells was significantly increased ((1.00±0.25) vs (9.70±2.63), P<0.01). In H/R-induced cells, transfection of miR-878 inhibitor significantly increased cell viability ((46.67±3.00) vs (74.62±4.08), P<0.000 1), and decreased LDH release ((358.58±41.71) vs (179.09±15.59), P<0.000 1) and cell apoptosis rate ((43.41±0.72) vs (27.42±4.48), P<0.01). At the same time, downregulation of miR-878 expression significantly inhibited DRP1-mediated mitochondrial overdivision and mtROS production ((6.60±0.57) vs (4.32±0.91), P<0.000 1). The mechanism study showed that miR-878 could target and bind Pim1 and inhibit the expression level of Pim1 ((1.00±0.13) vs (0.38±0.03), P<0.01). Rescue experiments confirmed that down-regulation of Pim1 expression significantly reversed the anti-injury effect of miR-878 inhibitor in H9c2 cells (P<0.01), promoted mitochondrial overdivision and mtROS production ((1.00±0.12) vs (2.41±0.12), P<0.01), and decreased the expression level of p-DRP1 ((1.00±0.15) vs (0.59±0.06), P<0.05).Conclusion The present study demonstrates that miR-878 expression is upregulated in H9c2 cardiomyocytes subjected to H/R injury. Inhibition of miR-878 expression alleviates H/R-induced cardiomyocyte damage. Notably, downregulation of miR-878 significantly inhibits DRP1-mediated mitochondrial fission and mitigates mtROS production. Mechanistically, miR-878 targets and binds to the 3"-UTR of the Pim1 gene, thereby suppressing Pim1 protein expression. Collectively, these findings suggest that under H/R conditions, miR-878 promotes excessive mitochondrial fragmentation through DRP1 activation by targeting Pim1, ultimately contributing to cardiomyocyte injury. Modulation of the miR-878/Pim1 axis may represent a potential therapeutic strategy for mitigating MI/R-induced cardiac damage.