The P-V-T equation of state (EOS) of gold is the most frequently used pressure calibration standard in high-P-T in situ experiments. Empirically proposed EOS models, however, severely scatter under high-P-T conditions, which is a serious problem for studies of the deep Earth. In this study, the EOS of gold is predicted using a first-principles electronic structure calculation method without any empirical parameters. The calculated thermoelastic properties of gold compare favorably to experimental data at ambient conditions so that BT0 and B'T0 are 166.7 GPa and 6.12, respectively. Up to V/Va = 0.7, the calculated Gr¿neisen parameter of gold depends on volume according to the function γ/γa = (V/Va)ζ with γa of 3.16 and ζ of 2.15. On the basis of these data, the validity of previous EOS models is discussed. It is found that the present ab initio EOS provides a 1.3 GPa higher pressure than Anderson's scale at 23 GPa and 1800 K and largely reduces the discrepancy observed between conditions at the transition of Mg2SiO4 and the 660-km seismic discontinuity. However, a discrepancy of about 0.7 GPa still remains between the 660-km discontinuity and the postspinel transition.