As an advanced level of control in colloidal synthesis, it is highly desirable to create secondary structures of nanocrystals in a controllable manner for collective properties. Of particular interest is the generation of nanoislands of plasmonic metals (Ag and Au) at a high density around their pre‐existing primary nanocrystals, which may produce abundant hotspots for surface‐enhanced Raman scattering (SERS). Often such secondary structures are difficult to be achieved by direct crystal growth because a conformal growth is favorable due to the lattice match of these metals. Here, this challenge is overcome by developing a partial surface passivation strategy which can effectively shift the crystal growth mode from the “Frank–van der Merwe” mode to the “Volmer–Weber” mode, giving rise to nanoislands as a secondary structure on Au nanocrystals. The key to this strategy is the modification of the Au surface with Ag and subsequent adsorption of iodide at the Ag sites. Further deposition of Au on the modified surface leads to the formation of well‐defined Au–Ag alloy islands of a high density on Au nanocrystals, which exhibit excellent SERS activity. This partial surface passivation strategy is fundamentally important and may inspire further endeavors in pursuit of novel secondary nanostructures and intriguing properties.