The expansion of salinization of agricultural areas limits the production of crops of high economic importance such as soybeans. To attenuate the effects of salts on the plant and improve photosynthetic performance under salinity conditions, the application of a biostimulant based on seaweed extract (Ascophyllum nodosum (L)) and fulvic acids as a physiological enhancer was adopted. In the present study, we evaluated whether applications of the biostimulant at different phenological stages can reduce damage to the photosynthetic apparatus in soybean plants under salt stress. The experiment was conducted in a greenhouse and the design adopted was completely randomized in a double factorial, consisting of three applications (V3, V3/R1 and V3/R1/R4), two levels of salinity of the irrigation water (S0 - absence of salt and S1 - saline solution at 5.0 dS m-1 prepared with the salts NaCl, CaCl2.2H2O and MgCl2.6H2O in the ratio 7:2:1) with two additional controls without application of the biostimulant (with and without stress) and five repetitions. Soybean plants were irrigated daily with the solutions and a weekly depth of 25% higher than the demand of the culture was applied. The evaluations were carried out 49 days after sowing, evaluating the potential quantum efficiency of photosystem II (Fv/Fm), the effective quantum yield of PSII (ɸFSII), the photochemical (qP ) and non-photochemical (qN) quenching and the rate of electron transport (ETR). Plants subjected to irrigation with saline water showed reductions in the evaluated parameters, suggesting that the salts caused damage to the photosynthetic apparatus in the photochemical stage. The application of the biostimulant was effective in reducing damage to the photosynthetic apparatus, providing greater efficiency in dissipating excess energy and less reduction in ETR. The application that provided the best results was in V3/R1.