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Summary: Angiogenesis is driven by the coordinated collective branching of specialized leading “tip” and trailing “stalk” endothelial cells (ECs). While Notch-regulated negative feedback suppresses excessive tip selection, roles for positive feedback in EC identity decisions remain unexplored. Here, by integrating computational modeling with in vivo experimentation, we reveal that positive feedback critically modulates the magnitude, timing, and robustness of angiogenic responses. In silico modeling predicts that positive-feedback-mediated amplification of VEGF signaling generates an ultrasensitive bistable switch that underpins quick and robust tip-stalk decisions. In agreement, we define a positive-feedback loop exhibiting these properties in vivo, whereby Vegf-induced expression of the atypical tetraspanin, tm4sf18, amplifies Vegf signaling to dictate the speed and robustness of EC selection for angiogenesis. Consequently, tm4sf18 mutant zebrafish select fewer motile ECs and exhibit stunted hypocellular vessels with unstable tip identity that is severely perturbed by even subtle Vegfr attenuation. Hence, positive feedback spatiotemporally shapes the angiogenic switch to ultimately modulate vascular network topology. : In angiogenesis, tight control of endothelial tip cell selection and migration drives new blood vessel branching. Page et al. show that VEGF-signaling-mediated positive feedback shapes the timing, magnitude, and robustness of tip cell identity decisions. Moreover, they identify a key modulator of VEGF-mediated positive feedback, Tm4sf18, which temporally controls angiogenesis in vivo. Keywords: angiogenesis, endothelial cell, tip cell, tetraspanin, positive feedback, lateral inhibition