Lead-free 0.96[Bi1/2(Na0.84K0.16)1/2(Ti(1−x)Nbx)O3]–0.04SrTiO3 (BNKTN–ST, with x = 0–0.030) ceramics were synthesized by a conventional solid-state reaction technique. Polarization and strain hysteresis loops indicated a significant disruption of ferroelectric order accompanied by an enhanced field-induced strain (S = 0.438%) with a high normalized strain Smax/Emax of 876 pm/V at 5 kV/mm. Their reproducibility was confirmed by the fabrication of a 10-layered stack-type multilayer actuator, which demonstrated a normalized strain Smax/Emax of 720 pm/V.