Vascular endothelial growth factor (VEGF) acting through VEGF receptor 2 (VEGFR2) on endothelial cells (ECs) is a key regulator of angiogenesis, a process essential for wound healing and tumor metastasis. Rap1a and Rap1b, 2 highly homologous small G proteins, are both required for angiogenesis in vivo and for normal EC responses to VEGF. Here we sought to determine the mechanism through which Rap1 promotes VEGF-mediated angiogenesis. Using lineage-restricted Rap1-knockout mice we show that Rap1-deficiency in endothelium leads to defective angiogenesis in vivo, in a dose-dependent manner. Using ECs obtained from Rap1-deficient mice we demonstrate that Rap1b promotes VEGF-VEGFR2 kinase activation and regulates integrin activation. Importantly, the Rap1b-dependent VEGF-VEGFR2 activation is in part mediated via integrin α(v)β(3). Furthermore, in an in vivo model of zebrafish angiogenesis, we demonstrate that Rap1b is essential for the sprouting of intersomitic vessels, a process known to be dependent on VEGF signaling. Using 2 distinct pharmacologic VEGFR2 inhibitors we show that Rap1b and VEGFR2 act additively to control angiogenesis in vivo. We conclude that Rap1b promotes VEGF-mediated angiogenesis by promoting VEGFR2 activation in ECs via integrin α(v)β(3). These results provide a novel insight into the role of Rap1 in VEGF signaling in ECs.