Advances in electrochemical devices have been primarily driven by the discovery and development of electrolyte materials. However, finding an electrolyte that exhibits both high ionic conductivity and chemical and electrochemical stability remains a significant challenge. Here, we developed such an electrolyte with heavily Sc-doped BaSn0.3Sc0.7O3H0.66. The resulting proton conductivities in BaSn0.3Sc0.7O3H0.66 and BaTi0.2Sc0.8O3H0.74 exceed 0.01 Scm−1 at 300°C. The former maintains chemical stability for over 360 h under 0.98 atm of humidified CO2 and electrochemical stability for over 100 h during fuel cell operation, with a theoretical overpotential of 1.194 V and proton conductivity of 0.01 Scm−1 at 300°C. Heavy Sc doping into perovskites reduces the activation energy for proton diffusion, probably due to reduced proton trapping. Screening for softer oxides with maximum Sc solubility is a promising strategy for developing fast and stable proton-conducting electrolytes capable of operating at 300°C.