Using <001>-oriented Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ ferroelectric single crystals as a model material, the impact of mechanical confinements on polarization hysteresis, coercive field, and remanent polarization of relaxor-based piezocrystals is investigated. Comparative studies are made among rhombohedral and tetragonal single crystals, as well as a polycrystalline ceramic, under uniaxial and radial compressive pre-stresses. The dramatic changes observed are interpreted in terms of the piezoelectric effect and possible phase transitions for rhombohedral crystals, and ferroelastic domain switching and the piezoelectric effect for tetragonal crystals. Under radial compressive stresses, the coercive field for the rhombohedral crystal is observed to increase to 0.67 kV/mm and that for the tetragonal crystal is increased to 0.78 kV/mm. This is a 200% increase relative to the unstressed condition. The results demonstrate a general and effective approach to overcome the drawback of low coercive fields in these relaxor-based ferroelectric crystals, which could help facilitate widespread implementation of these piezocrystals in engineering devices.