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Electric‐field‐induced strain in piezoelectric materials, which has demonstrated broad applications, usually provides about 1% strain. A giant and reversible deformation as large as ≈5% in BiFeO3 thin film induced by electrochemistry is reported, where the large lattice change is induced by electrochemical reaction through absorption and desorption of oxygen. Prior to deformation, a precursor phase with projected Fe–Fe pairs is formed first under low voltage (1 V), and then a large lattice expansion is eventually achieved under a high voltage (≈18 V), which is reversible under negative voltage. It is found that the giant strain is due to the electrochemically induced migration of oxygen ion which leads to significant out‐of‐plane lattice expansion under positive voltage, resulting in the formation of a new oxygen‐deficient phase. Interestingly, the new oxygen‐deficient phase is capable of transforming back to the pristine structure by absorbing oxygen under negative voltage. The results provide a new route to realize giant and reversible deformation in oxides by oxygen migration. [ABSTRACT FROM AUTHOR]