We here demonstrate the fabrication of Ag nanoparticle (NP) assemblies on branched Cu₂O crystals through a galvanic replacement reaction process, where the branched Cu₂O morphology can be well maintained by controlling the reaction environment (acid introduced to initiate the reaction). Cu₂O was transferred into soluble Cu²⁺ species, which renders a clean surface of Ag NPs. The as-fabricated Ag NP assemblies can be used as promising SERS-active substrates for molecule detection, with a detection sensitivity of 10⁻⁹ mol L⁻¹ towards both target molecules, MB and RhB. We believe galvanic replacement using Cu₂O as a sacrificing template can also be applied to synthesize noble metal (Au, Pt, Pd) NPs or NP assemblies for various applications.