Vegetable plants are more sensitive to salt stress during the early growth stages, hence, the availability of poor-quality brackish water can be a big issue for the nursery vegetable industry. Microbial biostimulants may promote growth and vigor and counterbalance salt stress in mature plants. This study aimed to evaluate the application of plant growth-promoting microorganisms for improving salt tolerance of tomato seedlings irrigated with increasing salinity (0, 25, and 50 mM NaCl) during nursery growth. Two commercial microbial biostimulants were applied to the substrate before seeding: 1.5 g L^-1 of TNC Bactorr^S13 containing 1.3 x 10⁸ CFU g^-1 of Bacillus spp.; 0.15 g L^-1 of Flortis Micorrize containing 30 % of Glomus spp., 1.24 x 10⁸ CFU g^-1 of Agrobacterium radobacter, Bacillus subtilis, Streptomyces spp. and 3 x 10⁵ CFU g^-1 of Thricoderma spp.. Tomato seedlings suffered negative effects of salinity on plant height, biomass, shoot/root ratio, leaf number, leaf area, relative water content, and stomatal conductance. The use of the bacterial biostimulants modified seedling growth and its response to salt stress. They had a growth-promoting effect on the unstressed tomato seedlings increasing fresh and dry biomass accumulation, leaf number, and leaf area and were successful in increasing salinity tolerance of tomato seedlings especially when using Flortis Micorizze that enhanced salinity tolerance up to 50 mM NaCl. The inoculation of the substrate with microbial biostimulants could represent a sustainable way to improve tomato transplant quality and to use brackish water in vegetable nurseries limiting its negative effect on seedling growth.