Tetrahydrobiopterin ($BH_4$) has been known to be an essential cofactor for the activities of nitric oxide (NO) synthase and aromatic amino acid hydroxylases, which are involved in physiological and pathological processes. In the present study, we report that sepiapterin, the more stable form of $BH_4$ precursor, modulates vascular endothelial growth factor-A (VEGF-A)-induced cell proliferation and adhesion in human umbilical vein endothelial cells (HUVECs). The antiproliferative activity of sepiapterin in VEGF-A-treated HUVECs is associated with inhibition of the expression of cyclin-dependent kinases (Cdks) such as Cdk4 and Cdk2. Pretreatment with NO synthase inhibitor does not abrogate the ability of sepiapterin to inhibit VEGF-A-induced cell proliferation and adhesion, indicating that the suppressive effects of sepiapterin on VEGF-A-induced responses are mediated by NO-independent mechanism. Finally, we show that sepiapterin modulates VEGF-A-induced cell proliferation and adhesion through down-regulation of VEGF receptor-2 downstream signaling pathways. Taken together, these findings represent a novel function of sepiapterin in the regulation of angiogenesis, supporting further development and evaluation of sepiapterin as an antiangiogenic agent.