Altered vasculature and the resultant chaotic tumor blood flow lead to the appearance in fast-growing tumors of regions with gradients of oxygen tension and acute hypoxia (less than 1.4% oxygen). Due to its roles in tumorigenesis and resistance to therapy, hypoxia represents a problem in cancer therapy. Insufficient delivery of therapeutic agents to the hypoxic regions in solid tumors is recognized as one of the causes of resistance to therapy. This led to the development of hypoxia imaging agents, and the use of hypoxia-activated anticancer prodrugs. Here we show the first example of the hypoxia-induced siRNA uptake and silencing using a nanocarrier consisting of polyethyleneglycol 2000, azobenzene, polyethyleneimine (PEI)(1.8 kDa), and 1,2-dioleyl-sn-glycero-3-phosphoethanolamine (DOPE) units (the nanocarrier is referred to as PAPD), where azobenzene imparts hypoxia sensitivity and specificity. We report hypoxia-activated green fluorescent protein (GFP) silencing in vitro and its downregulation in GFP-expressing tumors after intravenous administration. The proposed nanoformulation represents a novel tumor-environment-responsive modality for cancer targeting and siRNA delivery.