Arteries and veins are anatomically, functionally and molecularly distinct. The current model of arterial–venous identity proposes that binding of vascular endothelial growth factor to its heterodimeric receptor—Flk1 and neuropilin 1 (NP-1; also called Nrp1)—activates the Notch signalling pathway in the endothelium, causing induction of ephrin B2 expression and suppression of ephrin receptor B4 expression to establish arterial identity^,,,. Little is known about vein identity except that it involves ephrin receptor B4 expression, because Notch signalling is not activated in veins; an unresolved question is how vein identity is regulated. Here, we show that COUP-TFII (also known as Nr2f2), a member of the orphan nuclear receptor superfamily, is specifically expressed in venous but not arterial endothelium. Ablation of COUP-TFII in endothelial cells enables veins to acquire arterial characteristics, including the expression of arterial markers NP-1 and Notch signalling molecules, and the generation of haematopoietic cell clusters. Furthermore, ectopic expression of COUP-TFII in endothelial cells results in the fusion of veins and arteries in transgenic mouse embryos. Thus, COUP-TFII has a critical role in repressing Notch signalling to maintain vein identity, which suggests that vein identity is under genetic control and is not derived by a default pathway.