We report a high-pressure Raman scattering investigation of (1−x)Pb(Mg_1∕3Nb_2∕3)O₃-xPbTiO₃ (PMN-PT_x) , which is considered as a prototype relaxor-based piezoelectric. The observed spectral changes are important and it is the pressure-induced appearance of a sharp band at roughly 370  cm⁻¹ for all investigated PMN-PT_x , that allows the construction of a partial x-p phase diagram which is characterized by a high-pressure non-cubic structure. This high-pressure structure is very similar if not identical to the high-pressure structure of PMN. Finally, we present a discussion of the high-wavenumber part of the Raman spectrum in terms of a two-mode behavior, which suggests a very local order around Ti atoms.