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