A lead-free solid solution between potassium sodium niobate and silver niobate, (1−x)K0.5Na0.5NbO3–xAgNbO3 (KNN-AN, with x = 0–0.36), has been prepared in the form of ceramics by solid state reaction under O2 atmosphere. The crystal chemical study shows that Ag+ ion diffuses into the KNN lattices to form a solid solution of perovskite structure with x up to 0.30. The substitution of Ag+ ion for (K0.5Na0.5)+ ion results in linear decreases in the ferroelectric Curie temperature (TC), from 420 °C in KNN down to 325 °C in 0.70KNN-0.30AN, and in the tetragonal to orthorhombic phase transition temperature (TT-O), from 190 down to 150 °C. The relative density of the KNN-AN ceramics reaches 94%. The optimum piezoelectric properties are found in the ceramics of 0.82KNN-0.18AN, with a piezocoefficient d33 = 186 pC/N, an electromechanical coupling factor kp = 42.5%, and a TC = 355 °C. For the ceramics with x = 0.24, the remanent polarization reaches a maximum value of Pr = 20 μC/cm2, with a reduced coercive field Ec = 6.4 kV/cm and a TC = 340 °C. It is shown that the (1−x)KNN-xAN ceramics exhibit piezoelectric properties which are comparable or superior to the hot-pressed KNN ceramics and other KNN-based systems. In particular, the piezoelectric properties exhibit very a good thermal stability up to high temperature, which is a significant improvement from other KNN-based piezoceramics.