Endosomal release is an efficiency-limiting step for many nonviral gene delivery vehicles. In this work, nonviral gene delivery vehicles were modified with a membrane-lytic peptide taken from the endodomain of HIV gp41. Peptide was covalently linked to polyethylenimine (PEI) and the peptide-modified polymer was complexed with DNA. The resulting nanoparticles were shown to have similar physicochemical properties as complexes formed with unmodified PEI. The gp41-derived peptide demonstrated significant lytic activity both as free peptide and when conjugated to PEI. Significant increases in transgene expression were achieved in HeLa cells when compared to unmodified polyplexes at low polymer to DNA ratios. Additionally, peptide-modified polyplexes mediated significantly enhanced siRNA delivery compared to unmodified polyplexes. Despite increases in transgene expression and siRNA knockdown, there was no increase in internalization or binding of modified carriers as determined by flow cytometry. The hypothesis that the gp41-derived peptide increases the endosomal escape of vehicles is supported by confocal microscopy imaging of DNA distributions in transfected cells. This work demonstrates the use of a lytic peptide for improved trafficking of nonviral gene delivery vehicles.