The effect of the excitation of surface plasmon polaritons at the silver – chalcogenide glass interface on photostimulated diffusion of silver into the chalcogenide was studied for the first time. To excite plasmons, a high-frequency aluminum diffraction grating with a period of 248.5 nm was used, onto which a two-layer Ag–As2S3 structure was deposited. It was found that the process of photostimulated diffusion of silver into the chalcogenide layer is accelerated (that is, the photosensitivity of such a structure increases) when the surface plasmon polariton is excited at the Ag – As2S3 interface during the exposure. The dynamics of photostimulated changes in the optical characteristics of the structure, including the initial stage of the photodiffusion process, was registered by recording the changes in the plasmon excitation characteristics with the exposure time.