We have investigated the electromechanical response of potassium sodium niobate (K0.5Na0.5NbO3 or KNN) thick films. The high-field strain hysteresis loops and weak-field converse piezoelectric d(33) coefficient of the films were measured and compared with those of KNN bulk ceramics under the same electric field conditions. The converse d(33) values of the thick films and bulk ceramics were equal to 82.5 and 138pm/V, respectively, at 0.4kV/mm. The fundamental difference between the piezoelectric response of the KNN films and the ceramics was studied in terms of the effective (clamped) piezoelectric d(33) coefficient. The reduction in the piezoelectric d(33) coefficient of the KNN films, resulting from the clamping by the substrate, was compared to lead-based ferroelectric thick films, including Pb(Zr,Ti)O-3 (PZT) and (1-x)Pb(Mg1/3Nb2/3)O-3-xPbTiO(3) (PMN-PT). We propose a possible explanation, based on the particular elastic properties of KNN, for the small relative difference observed between the clamped and unclamped (bulk) d(33) of KNN, in comparison with lead-based systems.