Lead-free piezoceramics have been an important research topic over the past two decades, aiming to replace Pb-based piezoceramics for sensor and actuator applications. In this study, LiNbO₃ (LN) doping was employed in (1−x)Bi_1/2(Na_0.8K_0.2)_1/2Ti_0.99Nb_0.01O₃–xLN ceramics near its morphotropic phase boundary (MPB). The composition with x = 0.02 exhibits a remarkable unipolar strain (S_uni) response, achieving a high S_uni of 0.57% and a large piezoelectric strain coefficient (d₃₃^*) of 952 pm/V under a moderate electric field of 60 kV/cm. This enhancement originates from local disorder induced by cosubstitutions at the A and B sites, which facilitates a transition from a nonergodic relaxor (NER) to an ergodic relaxor (ER) state. Comprehensive analysis indicates that the overall performance in electrostrain surpasses that of most reported lead-free ceramics. By investigating the state of polar nanoregions, including thermal evolution and the electric-field-induced phase transition, the mechanisms underlying the macroscopic strain response to the LN content have been revealed, providing valuable guidance for designing high-performance lead-free relaxor ferroelectrics for sensor and actuator applications.