The ferroelectric, dielectric, elastic, piezoelectric, and electromechanical properties of tetragonal Pb(In1/2Nb1/2)O3−Pb(Mg1/3Nb2/3)O3−PbTiO3 (PIN–PMN–PT) crystals were investigated. The single domain piezoelectric coefficients d33, d15, and d31 were found to be 530, 2350, and −200 pC/N, respectively, with electromechanical coupling factors k33, k15, and k31 being on the order of 0.84, 0.85, and 0.58. The mechanical quality factor Q for longitudinal mode was found to be >700, with high coercive field (Ec) being on the order of 10 kV/cm. The temperature and dc bias electric-field characteristics of single domain tetragonal PIN–PMN–PT crystals were also investigated. In contrast to [001] oriented domain engineered rhombohedral crystals, tetragonal PIN–PMN–PT crystals exhibited broader temperature usage range and higher thermal/electric field stability, with improved coercive field and mechanical quality factor.