Proton transport in nanochannels under humid conditions is crucial for the application in energy storage and conversion. However, existing materials, including Nafion, suffer from limited conductivity of up to 0.2 siemens per centimeter. We report a class of membranes assembled with two-dimensional transition-metal phosphorus trichalcogenide nanosheets, in which the transition-metal vacancies enable exceptionally high ion conductivity. A Cd0.85PS3Li0.15H0.15 membrane exhibits a proton conduction dominant conductivity of similar to 0.95 siemens per centimeter at 90 degrees Celsius and 98% relative humidity. This performance mainly originates from the abundant proton donor centers, easy proton desorption, and excellent hydration of the membranes induced by cadmium vacancies. We also observed superhigh lithium ion conductivity in Cd0.5PS3Li0.3 and Mn0.77PS3Li0.46 membranes.