The origin of the electric field-induced strain in the polycrystalline ceramic 0.92Bi_1/2Na_1/2TiO₃–0.06BaTiO₃–0.02K_1/2Na_1/2NbO₃was investigated usingin situhigh-resolution X-ray and neutron diffraction techniques. The initially existing tetragonal phase with pseudocubic lattice undergoes a reversible phase transition to a significantly distorted rhombohedral phase under electric field, accompanied by a change in the oxygen octahedral tilting froma⁰a⁰c⁺toa⁻a⁻a⁻and in the tilting angle. The polarization values for the tetragonal and rhombohedral phases were calculated based on the structural information from Rietveld refinements. The large recoverable electric field-induced strain is a consequence of a reversible electric field-induced phase transition from an almost nonpolar tetragonal phase to a ferroelectrically active rhombohedral phase.