In this study, novel pH-responsive polyion complex micelles (PICMs) were developed for the efficient delivery of nucleic acid drugs, such as antisense oligonucleotide (AON) and short interfering RNA (siRNA). The PICMs consisted of a poly(amidoamine) (PAMAM) dendrimer–nucleic acid core and a detachable poly(ethylene glycol)-block-poly(propyl methacrylate-co-methacrylic acid) (PEG-b-P(PrMA-co-MAA)) shell. The micelles displayed a mean hydrodynamic diameter ranging from 50 to 70 nm, a narrow size distribution, and a nearly neutral surface charge. They could be lyophilized without any additives and stored in dried form. Upon redispersion in water, no change in complexation efficiency or colloidal properties was observed. Entry of the micelles into cancers cells was mediated by a monoclonal antibody fragment positioned at the extremity of the PEG segment via a disulfide linkage. Upon cellular uptake and protonation of the MAA units in the acidic endosomal environment, the micelles lost their corona, thereby exposing their positively charged endosomolytic PAMAM/nucleic acid core. When these pH-responsive targeted PICMs were loaded with AON or siRNAs that targeted the oncoprotein Bcl-2, they exhibited a greater transfection activity than nontargeted PICMs or commercial PAMAM dendrimers. Moreover, their nonspecific cytotoxicity was lower than that of PAMAM. The pH-responsive PICMs reported here appear as promising carriers for the delivery of nucleic acids.