The tumour vascular microenvironment supports tumorigenesis not only by supplying oxygen and diffusible nutrients but also by secreting soluble factors that promote tumorigenesis. Here we identify a feedforward mechanism in which endothelial cells (ECs), in response to tumour-derived mediators, release angiocrines driving aberrant vascularization and glioblastoma multiforme (GBM) progression through a hypoxia-independent induction of hypoxia-inducible factor (HIF)-1α. Phosphorylation of profilin-1 (Pfn-1) at Tyr 129 in ECs induces binding to the tumour suppressor protein von Hippel-Lindau (VHL), and prevents VHL-mediated degradation of prolyl-hydroxylated HIF-1α, culminating in HIF-1α accumulation even in normoxia. Elevated HIF-1α induces expression of multiple angiogenic factors, leading to vascular abnormality and tumour progression. In a genetic model of GBM, mice with an EC-specific defect in Pfn-1 phosphorylation exhibit reduced tumour angiogenesis, normalized vasculature and improved survival. Moreover, EC-specific Pfn-1 phosphorylation is associated with tumour aggressiveness in human glioma. These findings suggest that targeting Pfn-1 phosphorylation may offer a selective strategy for therapeutic intervention of malignant solid tumours.