Simultaneous ultrasonic elastic wave velocity and in situ synchrotron X-ray measurements on grossular garnet were carried out up to 17 GPa and 1 650 K. P- and S-wave velocities and bulk and shear modulus showed linear pressure and temperature dependence. These data yielded a pressure derivative of the bulk modulus of 4.42(7) and a shear modulus of 1.27(3), which are in good agreement with those of garnets with variable chemical compositions. Temperature dependence of the bulk modulus of grossular (-1.36x10(-2)GPa/K) is also similar to that of other garnets, while the temperature dependence of the shear modulus of grossular (-1.11x10(-2)GPa/K) is higher than those of magnesium end-member garnets and pyrolitic garnet.