Phase structures and electrical properties of lead-free piezoelectric (1−x)(Bi0.5Na0.5)TiO3–x(K0.5Na0.5)NbO3 (BNT–xKNN) ceramics with 0.08 ≤ x ≤ 0.19 were systematically investigated. Results showed that a phase transition from a tetragonal to a pseudocubic phase occurred in this system, as KNN content increases. The addition of KNN shifted both the depolarization temperature Td and rhombohedral–tetragonal phase transition temperature TR-T to lower temperatures and tended to enhance the relaxor behavior of the ceramics, which was well explained by the microdomain–macrodomain transition theory with calculating criterion K. At x = 0.10–0.11, Td reached room temperature (RT), which accordingly induced an enhancement of the unipolar strain that peaks at a value of 0.22% was obtained. Furthermore, as the compositions (x = 0.12–0.15) have Td below RT, samples exhibited high electrostrictive response with large electrostrictive coefficient Q33 (0.017–0.019 m4/C2) and good thermostability comparable with that of traditional Pb-based electrostrictors.