Several grafted polypropylenimine dendrimers were synthesized by modifying either polypropylenimine (PPI) dendrimer generation 2 (G2) or generation 3 (G3) via 1.6-hexandioldiacrylate with branched oligoethylenimine 800Da (OEI) or PPI dendrimer G2. The resulting derivatives were characterized (1H NMR, GPC) and their biophysical properties such as DNA condensing ability, colloidal stability and hydrodynamic diameters were determined. All grafted dendrimers were able to efficiently compact DNA to nanosized polyplexes (100–200 nm) and exhibited an increased colloidal stability as compared to their unmodified counterparts. In vitro, grafted dendrimers resulted in much higher transfection levels as compared to the unmodified ones displaying alongside a clear structure–activity relationship regarding their transfection/toxicity profile. Transfection levels of OEI-grafted dendrimers were the highest, being similar or even higher as compared to standard polyethylenimines (linear and branched), demonstrating that the incorporation of ethylenimine moieties is the key factor contributing to this boosted transfection efficiency. None of the compounds resulted in polymer-induced erythrocyte aggregation. Upon i.v. injection of OEI-grafted dendrimer polyplexes into tumor-bearing mice transgene expression was predominantly found in the (subcutaneous) tumors. Importantly, the tumor gene expression levels significantly increased with the higher dendrimer core generation.