Abstract: With the accelerating development and utilization of saline-alkali land resources in China, the cultivation area for alfalfa (Medicago sativa L.) in salinized soils has been continuously expanding. Enhancing salt tolerance during the seedling stage is crucial for successful establishment of alfalfa in challenging environments. Therefore, this study established three salt stress concentrations (0.3%, 0.6%, and 0.9%) combined with three gibberellic acid (GA₃) seed soaking concentrations (50, 100, and 150 mg·L^–1) to investigate the alleviating effects and mechanisms of exogenous GA₃ on alfalfa under varying salt stress levels, while determining the optimal GA₃ concentrations corresponding to different salinity conditions. The results demonstrated that: Under 0.3% salt stress, maximum shoot length (21.13% increase) and root length (18.96% increase) were achieved at 150 mg·L^–1 GA₃, while 100 mg·L^–1 GA₃ significantly enhanced fresh weight (10.49% increase) and dry weight (10.56% increase) compared to the control. Under 0.6% salt stress, 100 mg·L^–1 GA₃ optimized shoot length (14.88% increase) and dry weight (8.84% increase), whereas 50 mg·L^–1 GA₃ produced the longest roots (18.82% increase). Under 0.9% salt stress, 50 mg·L^–1 GA₃ significantly improved shoot length (15.7% increase), root length (10.75% increase), and dry weight (0.36% increase). Mechanistically, optimal GA₃ concentrations alleviated salt stress by: 1) Enhancing antioxidant enzyme activities (superoxide dismutase, peroxidase, catalase). 2) Increasing endogenous phytohormone levels (auxin and salicylic acid). 3) Reducing stress markers — malondialdehyde (MDA), abscisic acid (ABA), chloride ions (Cl^–), and sodium ions (Na⁺). A distinct inverse proportional relationship was observed between optimal GA₃ concentrations and salt stress intensity. Specifically, under low salinity (0.3%), GA₃ concentrations of 100–150 mg·L^–1 were most effective. For moderate salinity (0.6%), 100 mg·L^–1 GA₃ proved optimal, while under high salinity (0.9%), a lower concentration of 50 mg·L^–1 GA₃ was sufficient to alleviate stress. This study confirmed that exogenous GA₃ seed soaking effectively mitigates salt-induced phytotoxicity and promotes alfalfa seedling growth, with superior efficacy under low-to-moderate salt stress compared to severe stress conditions. These findings provide both a theoretical foundation and practical guidance for improving alfalfa cultivation efficiency in saline-alkali regions.