The effect of hydrostatic pressure on piezoelectric materials is a crucial factor for underwater device design. In this work, the dielectric, elastic, and piezoelectric properties of Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 [PMNT (67/33)] ceramics were investigated under pressure from 0.1 to 455 MPa. The dielectric permittivity and losses were found to increase under hydrostatic pressure due to enhanced domain wall motion. With increasing hydrostatic pressure, the mechanical losses drastically increased, which can be attributed to the increase of both domain wall motion and the friction between pressure-transmitting fluid and piezoelectric vibrators. In addition, piezoelectric coefficients (d33, d31) and electromechanical coupling factors (k33, k31) were found to decrease with increasing pressure, owing to the pressure-induced depolarization for PMNT ceramics.