Nominally anhydrous minerals formed deep in the mantle and transported to the Earth’s surface contain tens to hundreds of ppm wt H[subscript 2]O, providing evidence for the presence of dissolved water in the Earth’s interior. Even at these low concentrations, H[subscript 2]O greatly affects the physico-chemical properties of mantle materials, governing planetary dynamics and evolution. The diffusion of hydrogen (H) controls the transport of H[subscript 2]O in the Earth’s upper mantle, but is not fully understood for olivine ((Mg, Fe)[subscript 2]SiO[subscript 4]) the most abundant mineral in this region. Here we present new hydrogen self-diffusion coefficients in natural olivine single crystals that were determined at upper mantle conditions (2 GPa and 750–900 °C). Hydrogen self-diffusion is highly anisotropic, with values at 900 °C of 10[superscript −10.9], 10[superscript −12.8] and 10[superscript −11.9] m[superscript 2]/s along ,  and  directions, respectively. Combined with the Nernst-Einstein relation, these diffusion results constrain the contribution of H to the electrical conductivity of olivine to be σ[subscript H] = 10[superscript 2.12]S/m·C[subscript H2O]·exp[superscript −187kJ/mol/(RT)]. Comparisons between the model presented in this study and magnetotelluric measurements suggest that plausible H[subscript 2]O concentrations in the upper mantle (≤250 ppm wt) can account for high electrical conductivity values (10[superscript −2]–10[superscript −1] S/m) observed in the asthenosphere.