We studied the effects of reflective supports on the surface-enhanced Raman scattering (SERS) by Au nanoparticles (NPs) placed on top of them. When Au NPs are placed on a flat Au or Pt thin film, the SERS intensity of 4-aminobenzenethiol is doubled from that of Au NPs on a Si wafer. This phenomenon can be explained, through finite difference time domain (FDTD) simulations, with a strongly enhanced electromagnetic (EM) field at the gap space between the Au NP and the metallic surface originating from the coupling of the localized surface plasmon of Au NPs with the reflected beam from the surface. When a nanostructured Pt thin film is used as the support, the SERS intensity is further enhanced. Interestingly, the SERS intensity in this case is higher than the sum of the intensities on Au NPs and the nanostructured Pt thin film, suggesting a synergistic effect between them. However, such enhancement cannot be explained by EM field strengths obtained by simulations. In this case, the explanation needs considerations on the distribution of analyte molecules on the available surfaces of SERS active regions.