The piezoelectric response of [001] poled domain engineered (1−x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 (PMN-PT) crystals was investigated as a function of composition and phase using Rayleigh analysis. The results revealed that the intrinsic (reversible) contribution plays a dominant role in the high piezoelectric activity for PMN-PT crystals. The intrinsic piezoelectric response of the monoclinic (MC) PMN−xPT crystals, 0.31≤x≤0.35, exhibited peak values for compositions close to R-MC and MC-T phase boundaries, however, being less than 2000 pC/N. In the rhombohedral phase region, x≤0.30, the intrinsic piezoelectric response was found to increase as the composition approached the rhombohedral-monoclinic (R-MC) phase boundary. The maximum piezoelectric response was observed in rhombohedral PMN-0.30PT crystals, being on the order of 2500 pC/N. This ultrahigh piezoelectric response was determined to be related to the high shear piezoelectric activity of single domain state, corresponding to an ease in polarization rotation, for compositions close to a morphotropic phase boundary (MPB). The role of monoclinic phase is only to form a MPB with R phase, but not directly contribute to the ultrahigh piezoelectric activity in rhombohedral PMN-0.30PT crystals. The extrinsic contribution to piezoelectric activity was found to be less than 5% for the compositions away from R-MC and MC-T phase boundaries, due to a stable domain engineered structure. As the composition approached MPBs, the extrinsic contribution increased slightly (<10%), due to the enhanced motion of phase boundaries.