We describe a new Brillouin spectrometer that has been installed on a synchrotron x-ray beamline for simultaneous measurements of sound velocities (by Brillouin scattering) and density (by x-ray diffraction). The spectrometer was installed at the 13-BM-D station (GSECARS) of the Advanced Photon Source. This unique facility has been tested in studies of transparent single crystal and polycrystalline materials at high pressure and temperature. The equation of state, acoustic velocities, and, hence, elastic moduli of materials as a function of pressure and temperature can now be determined without resort to a secondary pressure standard, such as the ruby fluorescence scale, or the equation of state of standard materials such as Au, Pt, or MgO, thus offering the potential to determine an absolute pressure scale. This article describes the design of the combined Brillouin-x-ray system and the first experimental results obtained. As a general-user facility, the system was designed to require minimal setup time and alignment of sensitive optics, run-time control and adjustments of optics from outside the experimental station, compatibility with powder and single crystal x-ray diffraction measurements, and no interference with other experimental techniques used on the beamline. To satisfy these requirements we adopted a novel optical design for the Brillouin system with a vertical scattering plane. Examples of measurements of acoustic velocities and elastic moduli on single crystal NaCl (B1) and polycrystalline NaCl (B2) at high pressure and single crystal velocities and elastic moduli of MgO at high temperature (to 600 °C) and at high pressure are presented.