It has been shown for the first time that during the formation of the Ag/Se thin-film structure, as a result of Ag diffusion into the Se film and the course of a solid-phase chemical reaction, a hybrid plasmonic material containing a core of metallic silver nanoparticles with a shell of semiconductor Ag₂Se nanoparticles in an amorphous selenium matrix is formed. It is shown that the extinction spectrum obtained by spectroscopic ellipsometry shows the presence of two maxima in the blue and red regions of the optical spectrum, corresponding to localized surface plasmon resonances. It was found that the non-annealed synthesized films have high values of the extinction coefficient k=5.0 at λ=883 nm and the refractive index n=7.2 at λ=1074 nm. Keywords: thin-film structures, solid-phase chemical reaction, hybrid nanoparticles, plasmon resonance.