We investigated the effects of exogenous melatonin on the osmotic regulation and antioxidant capacity of 4-year-old Ginkgo biloba seedlings under salt stress. There were three treatments, with low (50 mmol·L-1), medium (100 mmol·L-1), and high (200 mmol·L-1) NaCl stress. Leaves were sprayed and the soil was watered with melatonin solution (0, 0.02, 0.1, 0.5 mmol·L-1). The results showed that saline stress significantly inhibited the osmoregulation and antioxidant capacities of G. biloba seedlings. Application of exogenous melatonin at appropriate concentrations (0.02, 0.1 mmol·L-1) under salt stress could promote plant growth, reduce the rate of electrolyte leakage, decrease the content of flavonoids and malonic dialdehyde, and enhance peroxidase and superoxide dismutase activities in leaves. High concentration (0.5 mmol·L-1) of exogenous melatonin would aggravate the oxidative and osmotic stresses. The 0.02 and 0.1 mmol·L-1 exogenous melatonin alleviated osmotic stress and oxidative stress in G. biloba seedlings under salt stress, while the 0.02 mmol·L-1 exogenous melatonin treatment had the best effect on NaCl stress alleviation. Ground diameter, branch width, branch length, electrolyte leakage rate, superoxide dismutase activity, and flavonoids content could be used as the key indices for rapid identification of the degree of salt stress in G. biloba seedlings.