Ferroelectric and piezoelectric properties of (001) 0.72PbMg1/3Nb2/3O3–0.28PbTiO3 (PMN–28%PT) single crystals have been investigated from cryogenic temperatures to 475 K. PMN–28%PT is used as piezoelectric substrate, e.g., in multiferroic heterostructures. Electric field-induced phase transformations have been examined by electrical characterization including measurements of polarization loops, dielectric permitivitty, and the resistance change in La0.7Sr0.3MnO3 films deposited on the (001) face. The relaxor ferroelectric transition behavior was studied by means of time-dependent current measurements. A phase diagram is set up. PMN–28%PT is found to be at the border of the appearance of the monoclinc phase (MC) bridging the rhombohedral-tetragonal (R-T) transformation at higher PbTiO3 contents. Measurements of the lattice expansion reveal that a high piezoelectric effect persists down to low temperatures. Therefore, PMN–28%PT single crystals are found to be appropriate substrates for application of piezoelectric strain to thin films over a broad temperature range.