For systemic siRNA delivery into tumor tissues, a safe and efficient vehicle is strongly required. Therefore, we designed a block copolymer of detachable poly(ethylene glycol) (PEG) polycation bearing low pKa amines and hydrophobic moieties in the side chain to develop a smart siRNA complex possessing biocompatibility, high complex stability, and endosomal escaping functionality. A disulfide linkage (-SS-) was inserted as a linker between PEG and a cationic polyaspartamide derivative, poly{N-[N-(2-aminoethyl)-2-aminoethyl]aspartamide} (PAsp(DET)), with a flanking stearoyl moiety, where PAsp(DET) segment provides the excellent ability of endosome destabilization by direct interaction with the membrane. The resulting polymer, stearoyl PEG-SS-PAsp(DET), was confirmed to form the siRNA complex with an environment-responsive PEG palisade, which was detached from the complex surface under reductive conditions mimicking tumor tissues and cytoplasm because of the disulfide cleavage. The smart siRNA complex allowed significant gene silencing against cultured pancreatic cancer cells without considerable cytotoxicity and erythrocyte disruption, whereas such significant gene silencing was not observed in a control siRNA complex without the disulfide linkage. This enhanced gene silencing activity might be because of the enhanced cellular uptake and subsequent translocation of siRNA into cytoplasm facilitated by PEG detachment around and/or in the cancer cells. Further, intravital real-time confocal laser scanning microscopic observation revealed the effect of hydrophobic stearoyl modification on the stabilization of the siRNA complex for longevity in the blood. Significant in vivo gene silencing of the smart siRNA complex was achieved by systemic administration of vascular endothelial growth factor siRNA in a mouse model bearing a subcutaneous pancreatic tumor, leading to 40% regression in tumor growth. These results demonstrate the strong potential of stearoyl PEG-SS-PAsp(DET) as a vehicle for systemic delivery of siRNA in cancer therapy.