Lead-free (1−x)(K0.48Na0.52)NbO3–(x/5.15)K2.9Li1.95Nb5.15O15.3 (KNN–KLN100x, x=0–0.25) piezoelectric ceramics have been prepared by a conventional solid-state sintering technique. The effects of potassium lithium niobate (KLN) content on the phase structure and piezoelectric properties were investigated. The addition of KLN markedly increased the paraelectric cubic-ferroelectric tetragonal phase transition temperature (TC), but greatly shifted the polymorphic phase transition from the ferroelectric orthorhombic to the ferroelectric tetragonal phase (TO–T) to near room temperature. At room temperature, coexistence of the orthorhombic and tetragonal phases was identified at approximately 0.12≤x≤0.18 by the analysis of X-ray diffraction patterns and dielectric spectroscopy, which led to a significant enhancement of the piezoelectric properties. The ceramics with x=0.16 exhibited excellent piezoelectric properties: piezoelectric constant d33=235 pC/N, planar electromechanical coupling factor kp=41.6%, and Curie temperature TC=473°C. Moreover, they also have an excellent thermal stability in the severe aging test up close to their Curie temperatures. These results indicated that the KNN–KLN100x ceramics are very promising lead-free piezoelectric candidates, especially for high-temperature applications.