We determined the kinetics of Fe‐Mg interdiffusion in ringwoodite aggregates as a function of water content (up to ~6,000 wt. ppm H₂O) at 20 GPa and 1,373–1,673 K by the diffusion couple method. The dependence of Fe‐Mg interdiffusivity (D_Fe‐Mg) on Fe concentration was determined using the Boltzmann‐Matano method. The experimentally reported D_Fe‐Mg in ringwoodite within 0 ≤ X_Fe ≤ 0.1 could be fitted by the relation , where E* = (1 − X_Fe)E_Mg + X_FeE_Fe (E_Mg = 140 ± 5 kJ/mol, E_Fe = 4 ± 2 kJ/mol), D₀ =  × 10⁻¹⁰ m²/s, n = −0.21 ± 0.10, r = 0.25 ± 0.03, and α = −24 ± 4. The water content exponent r of 0.25 suggests a nonnegligible role of water in enhancing Fe‐Mg interdiffusion in ringwoodite. The length scale over which the chemical heterogeneities are homogenized by Fe‐Mg interdiffusion in the mantle transition zone is estimated to be only a few hundred meters even assuming the whole Earth age. Comparison between the conductivities predicted from Fe‐Mg interdiffusion and those obtained from magnetotelluric surveys suggests that around 0.1 wt.% water can account for the high conductivity anomalies (~10^−0.6–10⁻¹ S/m) observed in the lower part of the mantle transition zone.