In this study, lead-free 0.975[Bi0.5(Na1−xKx)0.5TiO3]–0.025BiAlO3 piezoelectric ceramics (BNKT–BA) with x ranging from 0 to 0.25 were fabricated using a conventional solid state reaction. The effects of potassium concentration (x) on the structure and electrical properties of the ceramics were investigated. The X-ray diffraction patterns revealed that the addition of K+ improved the crystal symmetry of the BNKT–BA ceramics and produced a rhombohedral–tetragonal morphotropic phase boundary (MPB) in the range of 0.15≤x≤0.20. The increase of the K+ concentration resulted in a decreased grain growth rate and promoted the formation of grains with a cubic shape. The BNKT–BA ceramics exhibited high remanent polarizations of 25μC/cm2 near the rhombohedral side of the MPB composition (x=0.15). The polarization and strain hysteresis loops demonstrated that an increased K+ concentration (x≥0.20) destabilized the ferroelectric order of the BNKT–BA ceramics, leading to degradation of the remanent polarization and coercive field. However, the destabilization of the ferroelectric order was accompanied by significant enhancements in the bipolar and unipolar strains. A large electric field-induced strain (S=0.33%) and a corresponding normalized strain (d33*=Smax/Emax=533 pm/V) were obtained on the tetragonal side of the MPB composition (x=0.22).