Cerium oxides have attracted much attention because of their uses in three-way catalysts and other catalyst applications. The redox reaction of cerium oxides, as the basis of their use as catalysts, usually takes place at high temperature (>600 K) and/or low oxygen partial pressure. There have been continuous efforts to lower the operating temperatures of cerium oxide further to improve the performance of the catalysts and reduce pollution under the cold-start condition. Here, we report a direct atomic-scale observation of a redox process in cerium oxides driven by an electrical field at ambient temperature. The dynamic changes taking place during the electrically driven redox reaction were imaged by in situ high-resolution transmission electron microscopy, where reversible phase transformations due to the migration of oxygen vacancies have been reproducibly achieved. These results could lead to the low-temperature operation of catalysts for the purification of automobile emissions of pollutants, oxygen generation, and intermediate-temperature solid oxide fuel cells.