The solid solution of (100 - x)%Pb(Mg1/3Nb2/3)O3-x%PbTiO3 (PMN-xPT) exhibits ultrahigh piezoelectric and dielectric properties near the morphotropic phase boundary compositions and, thus, has been extensively studied in recent years. Recently, 2.5 mol. % Sm-doped PMN-29PT polycrystalline ceramics were reported to possess the highest piezoelectric coefficients (∼1500 pC/N) among all reported piezoceramics, but the atomic-scale mechanisms for such high piezoelectric properties are not yet clear. In this paper, in situ X-ray diffraction and X-ray total scattering measurements during the application of an electric field, together with in situ total scattering measurement at different temperatures, were conducted for 2.5 mol. % Sm-doped PMN-29.5PT (2.5Sm-PMN-29.5PT). Both the largest field-induced strain and the piezoelectric response were found in the crystallites oriented with their 100PC directions parallel to the applied field. The local Pb displacement was analyzed using the reverse Monte Carlo method based on the pair distribution functions at different temperatures, where a temperature-induced directional change of Pb displacement was observed. Based on the experimental observations, a field-induced polarization rotation is suggested to be the dominant mechanism for the ultrahigh piezoelectricity of the 2.5Sm-PMN-29.5PT ceramic, while the ease of polarization rotation is possibly attributed to the temperature-induced metastable local monoclinic symmetries with their polar axes close to 111PC.