We report the experimental measurement of Fe-Mg interdiffusivity in olivine along the  crystallographic direction in a water-saturated environment at pressures of 0.1 to 6 GPa and temperatures between 1373 and 1450 K. The concentration of water-derived protons in olivine was controlled by varying the water fugacity. The oxygen fugacity was set by the Ni-NiO solid state reaction, while the activity of silica was controlled by the presence of orthopyroxene. In this work, we report diffusivity as a function of temperature, pressure, and water fugacity following the relation D̃Fe -Mg = Do(fH2O/ foH2O)r exp [-(Q + PV*)/RT] m2s-1, where log(Do) = (-14.8 ± 2.7), r = 0.9 ± 0.3, Q = 220 ± 60 kJ/mol, and V* = (16 ± 6) × 10-6m3/mol. The approximately linear increase in diffusivity with increasing water fugacity is consistent with incorporation of protons associated with octahedral cation vacancies to form defect complexes. Our results indicate that cation diffusion in water-saturated olivine is ∼50 times faster than under water-absent conditions at a pressure of 5 GPa and a temperature of 1373 K.