The environmental benefits of fuel cells and electrolyzers have become increasingly recognized in recent years. Fuel cells and electrolyzers that can operate at intermediate temperatures (300–450 °C) require, in principle, neither the precious metal catalysts that are typically used in polymer‐electrolyte‐membrane systems nor the costly heat‐resistant alloys used in balance‐of‐plant components of high‐temperature solid oxide electrochemical cells. These devices require an electrolyte with high ionic conductivity, typically more than 0.01 S cm−1, and high chemical stability. To date, however, high ionic conductivities have been found in chemically unstable materials such as CsH2PO4, In‐doped SnP2O7, BaH2, and LaH3−2xOx. Here, fast and stable proton conduction in 60‐at% Sc‐doped barium zirconate polycrystal, with a total conductivity of 0.01 S cm−1 at 396 °C for 200 h is demonstrated. Heavy doping of Sc in barium zirconate simultaneously enhances the proton concentration, bulk proton diffusivity, specific grain boundary conductivity, and grain growth. An accelerated stability test under a highly concentrated and humidified CO2 stream using in situ X‐ray diffraction shows that the perovskite phase is stable over 240 h at 400 °C under 0.98 atm of CO2. These results show great promises as an electrolyte in solid‐state electrochemical devices operated at intermediate temperatures.