Efficient intracellular processes including cytosolic release and unpackaging of siRNA from the carrier in the cytoplasm are efficiency-determining steps in achieving successful gene silencing. In this study, acid-degradable ketalized linear polyethylenimine (KL-PEI) was synthesized for efficient, intracellular target-specific, and biocompatible siRNA delivery. The siRNA/KL-PEI polyplexes resulted in much higher RNA interference efficiency than unmodified L-PEI via selective cytoplasmic localization of the polyplexes and efficient disassembly of siRNA from the polyplexes, which were promoted upon acid-hydrolysis of amino ketal linkages. Confocal laser scanning microscopy demonstrated that siRNA was efficiently disassembled from the siRNA/KL-PEI polyplexes that were selectively localized in the cytoplasm. On the contrary, siRNA and unmodified linear PEI were colocalized in both the cytoplasm and the nucleus, and limited unpackaging of siRNA from the polyplexes was observed. In addition, ketalization further reduced the cytotoxicity of linear PEI but did not alter its serum-independent gene delivery efficiency. Therefore, KL-PEI is a promising nonviral vector for efficient and biocompatible siRNA delivery.