Lithium-ion-conducting solid electrolytes hold promise for enabling high-energy battery chemistries and circumventing safety issues of conventional lithium batteries. Achieving the combination of high ionic conductivity and a broad electrochemical window in solid electrolytes is a grand challenge for the synthesis of battery materials. Herein we show an enhancement of the room-temperature lithium-ion conductivity by 3 orders of magnitude through the creation of nanostructured Li3PS4. This material has a wide electrochemical window (5 V) and superior chemical stability against lithium metal. The nanoporous structure of Li3PS4 reconciles two vital effects that enhance the ionic conductivity: (1) the reduction of the dimensions to a nanometer-sized framework stabilizes the high-conduction beta phase that occurs at elevated temperatures, and (2) the high surface-to-bulk ratio of nanoporous beta-Li3PS4 promotes surface conduction. Manipulating the ionic conductivity of solid electrolytes has far-reaching implications for materials design and synthesis in a broad range of applications, including batteries, fuel cells, sensors, photovoltaic systems, and so forth.