The functional significance of the overexpression of unmutated TAp73, a homologue of the tumour suppressor p53, in multiple human cancers is unclear, but raises the possibility of unidentified roles in promoting tumorigenesis. We show here that TAp73 is stabilized by hypoxia, a condition highly prevalent in tumours, through HIF-1α-mediated repression of the ubiquitin ligase Siah1, which targets TAp73 for degradation. Consequently, TAp73-deficient tumours are less vascular and reduced in size, and conversely, TAp73 overexpression leads to increased vasculature. Moreover, we show that TAp73 is a critical regulator of the angiogenic transcriptome and is sufficient to directly activate the expression of several angiogenic genes. Finally, expression of TAp73 positively correlates with these angiogenic genes in several human tumours, and the angiogenic gene signature is sufficient to segregate the TAp73^Hi- from TAp73^Low-expressing tumours. These data demonstrate a pro-angiogenic role for TAp73 in supporting tumorigenesis, providing a rationale for its overexpression in cancers.