Polycrystalline, cubic 13C diamond was synthesized from amorphous carbon in the Kawai-type multianvil apparatus at 21 GPa and at a temperature greater than 2350°C. The polycrystalline diamond was homogeneous with a small grain size (10–20 μm) and a sharp Raman peak, and thereby was suitable as a pressure sensor for the experiments in a hydrothermal diamond anvil cell. Pressure- and temperature-dependence of the Raman shift of the synthesized 13C diamond was investigated in situ at simultaneous high pressures and high temperatures in the hydrothermal diamond anvil cell, using the ruby fluorescence line, quartz Raman shift and H2O phase transitions as pressure references. It was observed that the frequency shift with pressure is independent of temperature and vice versa up to 500°C and 4.2 GPa. The present study indicates that the 13C diamond Raman shift can be used for pressure determination with an accuracy better than ±0.3 GPa under the conditions examined.