We find rhombohedral-tetragonal morphotropic phase boundary showing high piezoelectricity in relaxor ferroelectric Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 subjected to hydrostatic pressure based on a Landau-Ginzburg-Devonshire phenomenological modeling. Room-temperature Curie transitions driven by moderate pressures were investigated by examining evolution of free energy landscape and equations for determining critical pressures are derived. Monoclinic state was induced when applying [111]-oriented electric field to originally metastable rhombohedral state coexisting with the pressure-stabilized tetragonal state within a narrow pressure window. High piezoelectricity found at the morphotropic phase boundary can be further enhanced under electric field.