One function of phosphoinositide 3-kinase α (PI3Kα), which generates the lipid second messenger phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)], is its regulation of angiogenesis in the developing embryo and in pathological situations. ARAP3 is a PtdIns(3,4,5)P(3)- and Rap-activated guanosine triphosphatase (GTPase)-activating protein (GAP) for the small GTPases RhoA and Arf6. Here, we show that deleting Arap3 in the mouse caused embryonic death in mid-gestation due to an endothelial cell-autonomous defect in sprouting angiogenesis. Explants taken at a developmental stage at which no defect was yet present reproduced this phenotype ex vivo, demonstrating that the defect was not secondary to hypoxia, placental defects, or organ failure. In addition, knock-in mice expressing an ARAP3 point mutant that cannot be activated by PtdIns(3,4,5)P(3) had angiogenesis defects similar to those of Arap3(-/-) embryos. Our work delineates a previously unknown signaling pathway that controls angiogenesis immediately downstream of PI3Kα through ARAP3 to the Rho and Arf family of small GTPases.