Plants are exposed in the environment to many unfavorable factors limiting their growth and yield. One of them is salt stress. Salinity degrades the environment, reducing the productive capacity of affected land. The aim of the study was to determine the effect of silicon foliar fertilization on photosynthetic efficiency and the activity of oat plants (Avena sativa L.) under salt stress. Plants grown in a pot experiment were subjected to soil treatment with sodium chloride (NaCl) at a concentration of 200 mM. Silicon was applied as a foliar fertilizer containing 200 g∙L^-1 SiO₂ in three concentrations (0.05%, 0.1%, 0,2%). Silicon was applied twice. Physiological measurements were carried out on the second and seventh day after each application. At the final stage of the experiment, a visual assessment of the plants' condition was carried out and the amount of fresh mass (FM) of the above-ground part was determined. High salt concentrations in the soil negatively affected the growth and development of plants compared with control plants. Si addition markedly changed the relative chlorophyll content in leaves (CCl) and selected chlorophyll fluorescence parameters (F_v/F_m, F_v/F₀, RC/ABS and PI) and gas exchange parameters (P_N, E, g_s, and C_i). Furthermore, fresh weight of plants increased with silicon treatments. The use of silicon fertilizer at a concentration of 0.1 and 0.2% had a positive effect on plants for longer than the concentration of 0.05%. Results indicated that the exogenous application of silicon improved tolerance of oat to salinity.