The fabrication of functional surfaces based on arrangements of metallic nanoparticles (MNPs) is challenging for biomedical, optoelectronic, and catalytic applications. Herein, the fabrication of 3D nanostructured microfibers is reported by the controlled cooperative precipitation of MNPs on electrospun polymer fiber mats, at the critical solution temperature of a thermoresponsive copolymer coating the MNPs surface. To obtain the metallic decorated surfaces, several factors must be taken into consideration, such as the previous functionalization of the metallic nanoparticle surface and the selection of a suitable polymer concentration, pH, and temperature. The results show that the plasmonic properties of the prepared surfaces depend on the solution temperature. Among potential applications, the high specific area of the resulting Au@polymer@fiber nanocomposites is ideal to be exploited for catalytic applications. Thus, the reduction of 4-nitrophenol has been studied as a model.