We develop an interesting route to prepare new sponge-like Au-Ag alloy nanocubes (NCs) with controlled porosity and atomic percentage through an interparticle alloying and dealloying process. Au@Ag NCs were first synthesized using Au nanooctahedra as initial seeds. Then, the Au@Ag NCs were covered with SiO2 and thermally annealed, forming solid Au-Ag alloy NCs with a SiO2 layer. After removing the majority of the SiO2 layer and leaching less-stable Ag from solid Au-Ag alloy NCs, uniform sponge-like porous Au-Ag alloy NCs were obtained. In this process, SiO2 not only prevents fusion between adjacent Au@Ag NPs under thermal annealing, but also directs the final shape of sponge-like Au-Ag alloy NPs with a cubic shape as a template. Thanks to the high-density "hotspots'' in nanopores, sharp corners and edges, and a synergistic effect between Au and Ag species, such sponge-like Au-Ag alloy NCs showed excellent SERS performance with an enhancement factor of similar to 10(8), which can effectively detect 4-aminothiophenol (4-ATP) at a concentration as low as 1 x 10(-10) M. This strategy is universal and it can be extended to prepare sponge-like Au-Ag alloy NPs with different accurate shapes. Such spongelike nanoporous alloy NPs have many potential applications such as in plasmonics, SERS, drug delivery, photothermal therapy, and catalysis systems.