Approximately 50% of cancer patients receive radiation treatment, either alone or in combination with other therapies. Tumor hypoxia has long been associated with resistance to radiation therapy. Moreover, the expression of hypoxia inducible factors HIF1α and/or HIF2α correlates with poor prognosis in many tumors. Recent evidence indicates that HIF1α expression can enhance radiation-induced apoptosis in cancer cells. We demonstrate here that HIF2α inhibition promotes tumor cell death and, in contrast to HIF1α, enhances the response to radiation treatment. Specifically, inhibiting HIF2α expression augments p53 activity, increases apoptosis, and reduces clonogenic survival of irradiated and non-irradiated cells. Moreover, HIF2α inhibition promotes p53-mediated responses by disrupting cellular redox homeostasis, thereby permitting reactive oxygen species (ROS) accumulation and DNA damage. These results correlate with altered p53 phosphorylation and target gene expression in untreated human tumor samples and show that HIF2α likely contributes to tumor cell survival including during radiation therapy.