The water stability of the fast lithium ion conducting glass–ceramic electrolyte, Li1+x+y Al x Ti2−x Si y P3−y O12 (LATP), has been examined in distilled water, and aqueous solutions of LiNO3, LiCl, LiOH, and HCl. This glass–ceramics are stable in aqueous LiNO3 and aqueous LiCl, and unstable in aqueous 0.1M HCl and 1M LiOH. In distilled water, the electrical conductivity slightly increases as a function of immersion time in water. The Li–Al/Li3−x PO4−y N y /LATP/aqueous 1M LiCl/Pt cell, where lithium phosphors oxynitrides Li3−x PO4−y N y (LiPON) are used to protect the direct reaction of Li and LATP, shows a stable open circuit voltage (OCV) of 3.64V at 25°C, and no cell resistance change for 1 week. Lithium phosphors oxynitride is effectively used as a protective layer to suppress the reaction between the LATP and Li metal. The water-stable Li/LiPON/LATP system can be used in Li/air secondary batteries with the air electrode containing water.