Grain growth in 8 mol% Y2O3-stabilized zirconia ceramics (8YSZ) under an electric current has been investigated. Enhanced grain growth on the cathode side starts at 1150°C, well below the conventional sintering temperature, while grain growth is dormant on the anode side until 1400°C. In fully dense samples, the grain size undergoes an abrupt transition, differing by a factor of more than 10 on the two sides. Porous samples also experience faster densification on the cathode side, but grain growth is postponed until full density is first reached. Estimated grain boundary diffusivity on the cathode side has an apparent activation energy about 1 eV lower than that of normal grain boundary diffusion. These results are attributed to supersaturated oxygen vacancies accumulated on the cathode side, causing cation reduction that lowers their migration barrier.