Rationale: Cardiac microvascular leakage and inflammation are triggered during myocardial infarction (MI) and contribute to heart failure. Hypoxia-inducible factor 2α ( Hif2α ) is highly expressed in endothelial cells (ECs) and rapidly activated by myocardial ischemia, but its impact in microvascular endothelial barrier function during MI is unclear. Objective: To test our hypothesis that the expression of Hif2α in ECs regulates cardiac microvascular permeability in infarcted hearts, which is through its binding partner aryl hydrocarbon nuclear translocator. (ARNT). Methods and Results: Experiments were conducted with mice carrying an inducible EC-specific Hif2α -knockout ( ecHif2α -/- ) mutation, with mouse cardiac microvascular endothelial cells (CMVECs) isolated from the hearts of ecHif2α -/- mice after the mutation was induced, and with human CMVECs and umbilical-vein endothelial cells transfected with ecHif2α siRNA. After MI induction, echocardiographic assessments of cardiac function were significantly lower, while measures of cardiac microvascular leakage (Evans blue assay), plasma IL6 levels, and cardiac neutrophil accumulation and fibrosis (histology) were significantly greater, in ecHif2α -/- mice than in control mice, and RNA-sequencing analysis of heart tissues from both groups indicated that the expression of genes involved in vascular permeability and collagen synthesis was enriched in ecHif2α -/- hearts. In cultured ECs, ec Hif2α deficiency was associated with declines in endothelial barrier function (electrical cell impedance assay) and the reduced abundance of tight-junction proteins, as well as an increase in the expression of inflammatory markers, all of which were largely reversed by the overexpression of ARNT. We also found that ARNT, but not Hif2α, binds directly to the IL6 promoter and suppresses IL6 expression. Conclusions: Endothelial HIF-2a protects from hypoxia-induced cardiac microvascular barrier dysfunction, promotes inflammation damage and represents a potential therapeutic target for cardioprotection, and prevention of fibrosis following acute ischemic injury.