Hypoxia-inducible factor-1α (HIF1α) is a crucial regulator of the cellular response to hypoxia through its regulation of genes that control erythropoiesis, angiogenesis and anaerobic metabolism. We have previously shown that HIF1α stability is regulated by SUMOylation under the hypoxic condition. However, how HIF1α became SUMOylated during hypoxia is still unknown. In this study we identify PIASy as a specific E3 ligase for hypoxia-induced HIF1α SUMOylation. Hypoxia promotes translocation of HIF1α to the nucleus to facilitate its binding to PIASy, enabling the conjugation of HIF1α by SUMO1. We further show that PIASy negatively regulates hypoxia-induced HIF1α stability and transactivation. Knocking down PIASy increases the angiogenic activity of endothelial cells. Moreover, we show an inverse relationship between expression of PIASy and tumor angiogenesis in colon cancer. Thus, we define an important role of PIASy in hypoxia signaling through promoting HIF1α SUMOylation.