The self-assembly of plasmonic nanoparticles (NPs) to form specific nanostructures is a powerful strategy for controlling their optical properties and is useful for developing new materials for chemical detection, diagnostic agents, and biomedical imaging. A method of nanoparticle assembly is reported that uses nanoparticle building blocks with complementary surface chemistries in aqueous media, that is, (1) a reversible addition-fragmentation chain transfer (RAFT) polymer with both end groups having strong interactions with gold surfaces and (2) the as-synthesized gold surface stabilized with citrate ions. When these building blocks are mixed, they assemble via a process that is analogous to stepwise polymerizations of small-molecule monomers with complementary reactivity (such as diacids (AA) and diamines (BB) to give polyamides (A–A–[B–B–A–A]n–B–B)). An advantage of this method is that it is possible to control the morphologies of the assemblies by varying the ratios of the two building blocks as well as the degree of polymer grafting. Morphologies such as dimers, supramolecular chains of nanoparticles, and branched assemblies are prepared, which have morphology-dependent optical properties.