Mixed ionic and electronic conducting (MIEC) perovskite oxides (ABO3) have a substantial role in carbon-neutral clean energy conversion and storage technologies. Owing to their favorable catalytic properties, high ionic and electronic conductivity, and chemical and redox stability, MIEC perovskite oxides are promising electrode materials in multiple applications, such as solid oxide fuel/electrolysis cells, oxygen transport membranes, metal–air batteries, electrochemical sensors, and electrocatalysts for water splitting. Here, taking (La0.8Sr0.2)0.95Cr0.5Fe0.5O3−δ (LSCrF8255) as a model MIEC perovskite oxide, we demonstrate that the oxygen mass transport properties are significantly enhanced under a humid reducing water vapor environment (pO2 < 1 mbar, pH2O = 30 mbar) by up to 4 orders of magnitude compared to those measured under dry (pO2 = 200 mbar) and wet (pO2 = 200 mbar, pH2O = 30 mbar) oxygen atmospheres. A 0.8 eV decrease in the activation energy for oxygen bulk diffusion was also found under water vapor, and a decrease in activation energy of 0.7 eV for water surface exchange compared to oxygen surface exchange was found. The mechanisms underpinning these enhancements were explored. Furthermore, LSCrF8255 has also exhibited a consistent surface composition evolution regarding Sr segregation and phase separation and an excellent bulk stability under both oxidizing and reducing environments at elevated temperatures.