Metallic periodic structures provide unique optical and photonic coupling effects. However, geometrically precise control of periodic structure is particularly challenging when studying scale at few nanometer. Here, a new class of highly ordered silver plasmonic supercrystals is successfully synthesized by means of a nanocasting process using ordered mesoporous silica as template. During the nanocasting via a chemical reduction, the diffusibility, viscosity, and strength of the reducing agents have an important influence on the formation of ordered superstructures. The silver superstructures demonstrate an excellent structural stability even after removing the silica template and a small nanogap of less than ≈2 nm between nanoparticles, which are very vital for light–matter coupling, giving rise to plasmonic hot spots. The silver plasmonic supercrystals exhibit an ultrahigh Raman enhancement with an average enhancement factor of ≈109 and an ultralow detection limit down to ≈0.1 × 10−15m for diverse chemical and biological molecules. This novel template strategy to fabricate periodic plasmonic nanostructures can lead to a new class of plasmonic nanostructures and open extraordinary potentials for diverse applications.