Synthetic peptides, including cyclic peptides and peptidomimetics, provide stability, protection, and long circulation times compared to free-circulating peptides. Dendritic structures with amino acids or peptides attached to the peripheral layer represent one form of peptidomimetics (i.e., a hybrid peptide/dendrimer construct) that has found use in biological applications. Constructing such dendritic structures from linear polymeric building blocks provides a further advantage of generating a highly ordered and defined structure in the nanoparticle size range. However, the rapid synthesis of such well-defined structures is still a challenge. In this work, we demonstrate that through modulating the copper activity concomitantly of the nitroxide radical coupling (NRC) and the azide–alkyne cycloaddition (CuAAC) reactions, polymeric dendrimers decorated with l-lysine on the periphery could be made rapidly in one pot at 25 °C. Three polymeric dendrimers were constructed with high purity (>94%) and with varying l-lysine density coated on the peripheral generation layer. The self-assembly of these dendrimers in water gave similar sizes to that found in organic solvents, suggesting that the aggregation number of dendritic structures in water was very low and possibly consisting of unimolecular micelles. The findings support the conclusion that the self-assembly of a dendritic architecture in water produces nanoparticles with predictable and well-controlled sizes. This synthetic methodology and the self-assembly properties represent an important step toward synthesizing peptide-decorated dendrimers targeted toward therapeutic applications.