Rationale: Transforming growth factor (TGF)-β was linked to abnormal vessel function and can mediate impairment of endothelial angiogenic responses. Its effect on microRNAs and downstream targets in this context is not known. Objective: To study the role of microRNAs in TGF-β–mediated angiogenic activity. Methods and Results: MicroRNA profiling after TGF-β treatment of endothelial cells identified miR-30a-3p, along with other members of the miR-30 family, to be strongly silenced. Supplementation of miR-30a-3p restored function in TGF-β–treated endothelial cells. We identified the epigenetic factor methyl-CpG-binding protein 2 (MeCP2) to be a direct and functional target of miR-30a-3p. Viral overexpression of MeCP2 mimicked the effects of TGF-β, suggesting that derepression of MeCP2 after TGF-β treatment may be responsible for impaired angiogenic responses. Silencing of MeCP2 rescued detrimental TGF-β effects on endothelial cells. Microarray transcriptome analysis of MeCP2-overexpressing endothelial cells identified several deregulated genes important for endothelial cell function including sirtuin1 (Sirt1). In vivo experiments using endothelial cell–specific MeCP2 null or Sirt1 transgenic mice confirmed the involvement of MeCP2/Sirt1 in the regulation of angiogenic functions of endothelial cells. Additional experiments identified that MeCP2 inhibited endothelial angiogenic characteristics partly by epigenetic silencing of Sirt1. Conclusions: TGF-β impairs endothelial angiogenic responses partly by downregulating miR-30a-3p and subsequent derepression of MeCP2-mediated epigenetic silencing of Sirt1.