<p>Spermidine (Spd) is a class of physiologically active polyamines synthesized during plant secondary metabolism. A large number of studies have shown that Spd plays an important role in plant salt tolerance, but its mechanism remains unclear. Hydrogen peroxide (H₂O₂), as an important signaling molecule, also plays a significant role in plant salt tolerance. In this study, cucumber was used as the experimental material to analyze the mechanism of exogenous Spd-induced salt tolerance in cucumber seedlings at the physiological, biochemical, and molecular levels. The results showed that the NaCl + Spd treatment maintained reactive oxygen species (ROS) homeostasis, reduced the degree of membrane lipid damage, increased proline and soluble protein levels, increased stomatal opening, and regulated the expression levels of genes related to chlorophyll synthesis and metabolism in cucumber seedlings compared with NaCl treatment. Meanwhile, the NaCl + Spd treatment could maintain ionic homeostasis, regulate the activities of related enzymes in the ascorbate (ASA)-glutathione (GSH) cycle, and increase the expression of mitogen-activated protein kinase (MAPK) cascade pathway-related genes. In contrast, the mitigating effect of exogenous Spd on plant salt stress was suppressed or almost eliminated by NaCl + Spd + diphenyleneiodonium (DPI, an inhibitor of H₂O₂ synthesis) or dimethylthiourea (DMTU, a scavenger of H₂O₂) treatments compared with NaCl + Spd treatments alone. In conclusion, our data suggest that spraying exogenous Spd under salt stress conditions reduces the damage caused by salt stress and that H₂O₂ mediates the increase in salt tolerance in cucumber by exogenous Spd.</p>