Abstract Surface roughness and nanostructures of metal thin films play a major role in Surface-Enhanced Raman Scattering (SERS) study due to their intrinsic electromagnetic (EM) hot spots, multipolar resonance, and other factors. Here, we present a low-cost and scalable fabrication route for a flexible, nanoporous Au–Ag bimetallic thin film on a plastic substrate, providing a practical alternative to complex lithography-based approaches. The substrate is engineered via thermal deposition of a 4 nm Au layer onto a predeposited Ag–TiO2/SnO2 nanoparticles (NPs) matrix. This process yields a percolated, conductive nanostructure with a root-mean-square roughness of ∼3.91 nm, high optical transparency (∼80%), and low sheet resistance (∼7.26 Ω/□). The resulting 4 nm Au/Ag–TiO2/SnO2 substrate exhibits exceptional SERS sensitivity, detecting rhodamine 6G (R6G) and vitamin B12 down to 1 pM and 1 nM, respectively. Statistical analysis confirms exceptional detection sensitivity, corresponding to approximately 2 molecules for R6G and 89 molecules for vitamin B12 within the laser spot. This enhanced performance is attributed to a synergistic mechanism combining dense EM hot spots within the porous network and efficient plasmon-induced charge transfer (PICT) from the Au–Ag fermi level and trap-assisted TiO2/SnO2 conduction band to the analyte LUMO states. These findings demonstrate a robust, flexible platform for ultratrace chemical sensing.