0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (PMN–PT) ferroelectric thin films with thickness ranging from 28 to 110 nm were sputter deposited onto LaNiO3/SiO2/Si substrates. Optical properties were determined by spectroscopic ellipsometry. We found B = 4.6 and λ0 = 209 nm, which is consistent for all PMN–PT samples with previous results shown in the literature. Nanoscale electromechanical activity was probed by using piezoresponse force microscopy in imaging and spectroscopic modes. Both piezoresponse images and local piezoloops recorded on each film highlighted an enhancement of piezoelectric vibration amplitude when the film thickness increased from 28 to 62 nm (∼1.06 to ∼1.34 mV), then saturation was observed for thicker films. This specific evolution was explained taking into account the low-permittivity interfacial Pb2Nb2O7 layer existing between bottom electrode and PMN-PT layer. Higher leakage current when thickness is decreasing was shown, which could also explain the particular behavior of the local electromechanical properties.