Solid-state batteries are a promising energy storage technology that can potentially offer both improved safety and energy density. The solid electrolyte is the defining feature and plays a significant role in the electrochemical performance of a solid-state cell, especially at room temperature. Herein, we report a series of glass-ceramic, sodium-deficient chloride solid electrolytes, NaxY0.25Zr0.75Cl3.75+x (0.25 x 0.875), possessing significantly improved ionic conductivities when compared to their stoichiometric counterpart, Na2.25Y0.25Zr0.75Cl6 (x = 2.25). By tuning both the sodium molar content and the sample’s crystallinity, the composition Na0.625Y0.25Zr0.75Cl4.375 (x = 0.625) was found to exhibit the highest Na+ conductivity of 0.4 mS cm−1 at room temperature. Furthermore, the relationship between composition, structure, and conductivity for these compositions in the NaCl−YCl3−ZrCl4 system was evaluated using a combination of X-ray diffraction (XRD), solid-state nuclear magnetic resonance spectroscopy (ss-NMR), and electrochemical impedance spectroscopy (EIS) techniques. Materials characterization reveals that sodium-deficiency (i.e., lower molar % of NaCl) results in reduced crystallinity and preferred occupancy of prismatic Na local environments. These combined factors contribute to a lower activation energy for Na+ hopping, an increased ionic conductivity, and improved electrochemical performance at both higher cycling rates and at room temperature.