The presence of leachable alkali ions, or their hydrated sites in the glass, is believed to be a determining factor for the interfacial water structure at the glass surface, influencing the surface properties of glass. The interfacial water structure on soda-lime silicate glass in humid ambience at room temperature was analyzed with sum-frequency-generation (SFG) vibration spectroscopy, which can probe the interfacial water layer without spectral interferences from the gas phase water. The soda-lime glass surface exposed to water vapor shows three sharp SFG peaks at 3200, 3430, and 3670 cm−1 in SFG, which is drastically different from the SFG spectra of the water layers on the fused quartz glass surface and the liquid water/air interface. The sharp peak at 3200 cm−1 is believed to be associated with the hydronium ions in the Na+-leached silicate glass surface. The 3200 cm−1 peak intensity varies with the relative humidity, indicating its equilibrium with the gas phase water. It is proposed that the hydronium ions in the Na+-leached sites produce compressive stress in the silicate glass surface; thus the growth of hydronium ions with increasing humidity might be responsible for the increased wear resistance of soda-lime glass surfaces in near-saturation humidity conditions.