Background Malic enzyme 1 (ME1) catalyzes malate to pyruvate and thus promotes glycolysis, playing a part in the Warburg effect. It also has a potential role in tumor progression, but its function in breast cancer remains to be fully clarified. This work aimed to investigate the prognostic value of ME1 and its possible mechanism in breast cancer.Methods We evaluated ME1 expression in 220 early breast cancer patients with tissue microarray-based immunohistochemistry and explored the relationships between ME1 expression and clinicopathological features. Survival analyses were further performed to determine its prognostic value. The public database was used to confirm tissue microarray results. Moreover, we profiled ME1 expression in breast cancer cell lines via western blotting, and then assessed it in cell viability and motility via Cell counting kit-8 (CCK-8), colony formation, transwell migration and invasion assays. Reactive oxygen species (ROS) was detected by dihydroethidium (DHE) and 2’,7’-Dichlorodihydrofluorescein diacetate (DCFH-DA).Results In breast cancer tissues, high ME1 expression was significantly associated with larger tumor size, more lymph node metastasis and more extensive lymph-vascular invasion. Survival analysis showed high ME1 expression was significantly correlated with worse recurrence free survival (RFS). Multivariate analysis further identified high ME1 expression as an independent prognostic factor for RFS, which was confirmed by mRNA results in the public database. In vitro , human epidermal growth factor receptor-2 positive and triple negative breast cancer cell lines showed higher expression of ME1, while Luminal cell lines showed lower expression of ME1. Upregulation of ME1 by transfecting MCF-7 cells with virus vector remarkably enhanced viability and motility, epithelial-mesenchymal transition (EMT), and decreased ROS levels, whereas knockdown of this gene in MDA-MB-468 cells produced totally opposite effects as expected. More important, when pretreated with hydrogen peroxide, an oxidizing agent, MCF-7 cells overexpressing ME1 lost its motility, whereas MDA-MB-468 cells with knock-down of ME1 restored its motility when pretreated with N-acetyl cysteine, an antioxidant.Conclusions To our knowledge, these clinical and experiment work first suggested that ME1 may be a potential therapeutic target for breast cancer metastasis, and its biological effect is mainly controlled by manipulating ROS.