AbstractFabricating three‐dimensional (3D) hierarchical noble‐metal particles by spontaneous redox reactions between graphene and noble‐metal salts still remains a great challenge. Herein, the fact that graphene oxide (GO) itself acts as both a platform for grafting polymer brushes and a reducing agent to reduce [Ag(NH3)2]+ ions is taken advantages of. 3D flower‐like Ag mesoparticles (Ag mesoflowers, Ag MFs) with tunable size and shapes can spontaneous grow on poly(4‐vinylpyridine) brushes‐grafted‐graphene oxide (P4VP‐g‐GO) films in Ag(NH3)2OH solution without the use of any additional reducing agent. The residual Ag(NH3)2OH on 3D Ag MFs surface can be further reduced by NaBH4, causing abundant nanoporosities over the entire Ag MFs. The resulting Ag nanoporous MFs (Ag NMFs) with larger surface‐to‐volume ratio and higher nanoscale roughness exhibit ultrasensitivity in surface‐enhanced Raman spectroscopy (SERS) detection, and the detection limit for 4‐aminothiophenol is as low as 10−13 m.