A strong interest exists in developing surface-enhanced Raman spectroscopy (SERS) substrates that uniformly enhance Raman signals of chemical and biological molecules over large scales while reaching the detection limit of trace concentrations. Even though the resonant excitation of localized surface plasmons of single or assembled metallic nanoparticles used in SERS substrates can induce large electromagnetic fields, these substrates display a SERS activity which suffers from poor reproducibility, uniformity, and stability, preventing them from being reliable for applications. In this work, we have developed self-supported large scale Ag/Au bimetallic SERS-active substrate with a high density of nanoparticles and uniform hot spots. The resultant substrates are very stable under ambient conditions, providing unchanging Raman enhancement signals even after one year of fabrication, due to the protective Au shell on the bimetallic nanoparticles. The Ag/Au bimetallic substrate exhibits remarkable SERS enhancement for nonresonant molecules, permitting the detection of trace concentrations reaching 10–13 mol/L.