The influence of surface plasmons excited in a polycrystalline silver film on the tunneling current of a scanning tunneling microscope (STM) has been analyzed. The plasmons cause an additional flow of electrons from the tungsten tip to the silver surface on the order of up to 50 pA. This process is independent of the polarity of the applied bias voltage, thereby excluding effects of thermal expansion. The different nature of the ordinary tunneling current and the surface plasmon induced current is clearly revealed by their different dependence on the gap distance. The local distribution of the intensity of the surface plasmon induced signal reveals structures on a nanometer scale. Some of them are correlated to the surface topography.