Poly(amido ethylenimine) polymers, a new type of peptidomimetic polymer, containing multiple disulfide bonds (SS-PAEIs) designed to degrade after delivery of plasmid DNA (pDNA) into the cell were synthesized and investigated as new carriers for triggered intracellular gene delivery. More specifically, three SS-PAEIs were synthesized from Michael addition reactions between cystamine bisacrylamide (CBA) and three different ethylene amine monomers, i.e., ethylenediamine (EDA), diethylenetriamine (DETA), or triethylenetetramine (TETA). Complete addition reactions were confirmed by 1H NMR. The molecular weight, buffer capacity, and relative degree of branching for each SS-PAEI was determined by gel permeation chromatography (GPC), acid−base titration, and liquid chromatography−mass spectroscopy (LC-MS), respectively. Physicochemical characteristics of polymer/pDNA complexes (polyplexes) were analyzed by gel electrophoresis, particle size, and ζ-potential measurements. All three SS-PAEIs effectively complex pDNA to form nanoparticles with diameters less than 200 nm and positive surface charges of 32 mV. The in vitro gene transfer properties of SS-PAEIs were evaluated using mouse embryonic fibroblast cell (NIH3T3), primary bovine aortic endothelial cell (BAEC), and rat aortic smooth muscle cell (A7R5) lines. Interestingly, polyplexes based on all three SS-PAEIs exhibited remarkably high levels of reporter gene expression with nearly 20× higher transfection efficiency than polyethylenimine 25k. The high transfection efficiency was maintained in the presence of 10% serum in the transfection medium. Furthermore, confocal microscopy experiments using labeled pDNA indicated that polyplexes of SS-PAEI displayed greater intracellular distribution of pDNA as compared to PEI, most likely due to environmentally triggered release. Therefore, SS-PAEIs are a new class of transfection agents that facilitate high gene expression while maintaining a low level of toxicity.