The modification of a commercial nanofiltration (NF) membrane (TFC-S) with shape-persistent dendritic molecules is reported. Amphiphilic aromatic polyamide dendrimers (G1–G3) are synthesized via a divergent approach and used for membrane active layer modification by direct percolation. The permeate samples collected from the percolation experiments are analyzed by UV-visible spectroscopy to monitor the influence of dendrimer generations on percolation behavior and active layer modification. Further characterization of modified membranes by Rutherford backscattering spectrometry and atomic force microscopy techniques reveals a relatively low-level accumulation of dendrimers inside the original TFC-S NF membrane active layer and subsequent formation of a coating of pure aramide dendrimers on top of the active layer. A PES-PVA ultrafiltration membrane is used as a control membrane support (without an NF active layer) showing that structural compatibility between the dendrimers and support plays an important role in the membrane modification process. The performance of the modified TFC-S membrane is evaluated on the basis of the rejection abilities for a variety of water contaminants having a range of molecular size and chemistry. As the water flux is inversely proportional to the thickness of the active layer, the amount of dendrimers deposited for specific contaminants are optimized to improve the solute rejection while maintaining high water flux.