Salicylic acid (SA) and nitric oxide (NO) are two biologically active signaling molecules in plants. Both of these molecules play important roles in various aspects of plant growth and development, including seed germination, root growth, leaf elongation, stomatal movement, and respiration. As signaling molecules, SA and NO also participate in signal transduction in the responses to biotic and abiotic stresses. Previous studies have shown that the signaling pathways of SA and NO do not exist in isolation, but "cross-talk" with each other, and they show synergistic interactions in many physiological and resistance reactions. Scientists inspired by the "cross-talk" and synergistic interactions have produced research on the synergistic induction of signaling pathways to improve plant resistance using exogenous SA and sodium nitroprusside (SNP), a NO donor. In the last decade, much research has been conducted on use of exogenous SA or SNP to improve the salt resistance of plants. However, most of these studies have focused on the ability of either SA or NO to induce resistance, and on the physiological and biochemical mechanisms by which these compounds induce resistance. Few studies have focused on synergistic induction of defense pathways using both SA and NO to improve plant resistance to salt stress. In our experiments, we evaluated the synergistic effects of SA and NO on the resistance of tomato (Lycopersicon esculentum Mill. cultivar "Qin Feng Bao Guan") seedlings to salt stress. We applied SA and SNP alone or in combination to tomato seedlings, and measured the growth, photosynthesis, and ion distribution in the tomato seedlings under salt stress (100 mmol/L NaCl) in a nutrient solution culture system. The main findings of our research are as follows: (1) Single or combined applications of SA and SNP reduced the decreases in the levels of photosynthetic pigments (chlorophyll a, chlorophyll b, chlorophyll a+b and Carotenoids), the ratio of chlorophyll a and chlorophyll b, net photosynthetic rate, transpiration rate, stomatal conductance, instantaneous water use efficiency, apparent light energy use efficiency, and apparent CO₂ use efficiency in tomato seedlings under salt stress. These treatments also reduced the increase in the intercellular CO₂ concentration and the value of Carotenoids/chlorophyll a+b in leaves of tomato seedlings under NaCl stress. The strongest effect was observed after applying a combination of SA and SNP. (2) Under NaCl stress, the Cl^- and Na⁺ contents and the Na⁺/K⁺, Na⁺/Ca²⁺, Na⁺/Mg²⁺ ratios in leaf, stem, and root tissues of tomato seedlings were lower in plants treated with SA, SNP, or SA + SNP than in untreated salt-stressed tomato seedlings. The K⁺, Ca²⁺,and Mg²⁺ contents of plant tissues were improved to different extents by SA, SNP, or SA + SNP applications, but the latter treatment had the strongest effect. (3) Single or combined applications of exogenous SA and SNP effectively reduced the negative effects of NaCl stress on tomato seedling growth and development, and promoted the accumulation of leaf, stem, and root biomass. The combined application of SA and SNP had the strongest effect. The results of our research show that a combined application of exogenous SA and SNP has a synergistic effect on improving the salt tolerance of tomato seedlings.