Background and Purpose-Long noncoding RNA H19 is repressed after birth, but can be induced by hypoxia. We aim to investigate the impact on and underlying mechanism of H19 induction after ischemic stroke. Methods-Circulating H19 levels in stroke patients and mice subjected to middle cerebral artery occlusion were assessed using real-time polymerase chain reaction. H19 siRNA and histone deacetylase 1 (HDAC1) plasmid were used to knock down H19 and overexpress HDAC1, respectively. Microglial polarization and ischemic outcomes were assessed in middle cerebral artery occlusion mice and BV2 microglial cells subjected to oxygen-glucose deprivation. Results-Circulating H19 levels were significantly higher in stroke patients compared with healthy controls, indicating high diagnostic sensitivity and specificity. Moreover, plasma H19 levels showed a positive correlation with National Institute of Health Stroke Scale score and tumor necrosis factor-alpha levels. After middle cerebral artery occlusion in mice, H19 levels increased in plasma, white blood cells, and brain. Intracerebroventricular injection of H19 siRNA reduced infarct volume and brain edema, decreased tumor necrosis factor-alpha and interleukin-1 beta levels in brain tissue and plasma, and increased plasma interleukin-10 concentrations 24 hours poststroke. Additionally, H19 knockdown attenuated brain tissue loss and neurological deficits 14 days poststroke. BV2 cell-based experiments showed that H19 knockdown blocked oxygen-glucose deprivation-driven M1 microglial polarization, decreased production of tumor necrosis factor-alpha and CD11b, and increased the expression of Arg-1 and CD206. Furthermore, H19 knockdown reversed oxygen-glucose deprivation-induced upregulation of HDAC1 and downregulation of acetyl-histone H3 and acetyl-histone H4. In contrast, HDAC1 overexpression negated the effects of H19 knockdown. Conclusions-Our findings indicate that H19 promotes neuroinflammation by driving HDAC1-dependent M1 microglial polarization, suggesting a novel H19-based diagnosis and therapy for ischemic stroke.