Although it is well established that tumors initiate an angiogenic switch, the molecular basis of this process remains incompletely understood. Here we show that the miRNA miR-132 acts as an angiogenic switch by targeting p120RasGAP in the endothelium and thereby inducing neovascularization. We identified miR-132 as a highly upregulated miRNA in a human embryonic stem cell model of vasculogenesis and found that miR-132 was highly expressed in the endothelium of human tumors and hemangiomas but was undetectable in normal endothelium. Ectopic expression of miR-132 in endothelial cells in vitro increased their proliferation and tube-forming capacity, whereas intraocular injection of an antagomir targeting miR-132, anti–miR-132, reduced postnatal retinal vascular development in mice. Among the top-ranking predicted targets of miR-132 was p120RasGAP, which we found to be expressed in normal but not tumor endothelium. Endothelial expression of miR-132 suppressed p120RasGAP expression and increased Ras activity, whereas a miRNA-resistant version of p120RasGAP reversed the vascular response induced by miR-132. Notably, administration of anti–miR-132 inhibited angiogenesis in wild-type mice but not in mice with an inducible deletion of Rasa1 (encoding p120RasGAP). Finally, vessel-targeted nanoparticle delivery of anti–miR-132 restored p120RasGAP expression in the tumor endothelium, suppressed angiogenesis and decreased tumor burden in an orthotopic xenograft mouse model of human breast carcinoma. We conclude that miR-132 acts as an angiogenic switch by suppressing endothelial p120RasGAP expression, leading to Ras activation and the induction of neovascularization, whereas the application of anti–miR-132 inhibits neovascularization by maintaining vessels in the resting state.