Piezoelectric (K0.5Na0.5)NbO3 (KNN) and (K0.5Na0.5)(Nb0.7Ta0.3)O3 (KNNT) thin films were prepared via chemical solution deposition. An analysis of X-ray diffraction revealed that Ta5+ diffuses into the KNN to form a single perovskite structure. Compared to KNN films, KNNT films exhibited a low leakage current density due to their fine-grain nonporous structures. The partial substitution of Ta5+ for the B-site ion Nb5+ in the KNNT films decreased the Curie temperature (TC). This in turn led to the existence of a polymorphic phase transition near room temperature and further improvement in the piezoelectric properties. Lead-free KNNT films exhibited a well-saturated piezoelectric hysteresis loop with a effective piezoelectric coefficient (d33,eff) value of 61 pm/V, comparable to that of PZT thin films.