The objective of this study was to investigate whether the application of silicon (Si) ameliorates the detrimental effects of salinity stress on sainfoin (Onobrychis viciaefolia). Three-week-old seedlings were exposed to 0 and 100 mmol/L NaCl with or without 1 mmol/L Si for 7 days. The results showed that salinity stress significantly reduced plant growth, shoot chlorophyll content and root K<sup>+</sup> concentration, but increased shoot malondialdehyde (MDA) concentration, relative membrane permeability (RMP) and Na<sup>+</sup> concentrations of shoot and root in sainfoin compared to the control (no added Si and NaCl). However, the addition of Si significantly enhanced growth, chlorophyll content of shoot, K<sup>+</sup> and soluble sugars accumulation in root, while it reduced shoot MDA concentration, RMP and Na<sup>+</sup> accumulation of shoot and root in plants under salt stress. It is clear that silicon ameliorates the adverse effects of salt stress on sainfoin by limiting Na<sup>+</sup> uptake and enhancing selectivity for K<sup>+</sup>, and by adjusting the levels of organic solutes. The present study provides physiological insights into understanding the roles of silicon in salt tolerance in sainfoin.