We describe design and synthesis model of multidomain (modular) peptides (MDPs), which direct a reaction cascade coupling the synthesis and surface functionalization of gold nanoparticles (AuNPs) in a single step. The synthesis is achieved via simple mixing of the aqueous solutions of auric acid and MDPs at room temperature without the addition of any surfactants or toxic intermediate reagents. This method allows facile control over the nanoparticle size between ∼2–15 nm, which opens a practical window for biomedical applications. In contrast to the conventional citrate-mediated methods, peptide-mediated synthesis and stabilization provide increased colloidal stability to AuNPs. As a proof of this concept, we demonstrate active targeting of human breast adenocarcinoma cell line (MCF7) using the one-step-prepared engineered AuNPs. Overall, we propose a single-step, chemically greener, biologically safer method for the synthesis and surface functionalization of gold nanoparticles in a size-controlled manner. The chemical versatility of the MDP design broadens the applicability of this strategy, thereby emerging as a successful alternative for the currently available nanoparticle preparation technologies.