The influence of melt on the creep behavior of olivine-basalt aggregates under hydrous conditions has been investigated by performing a series of high-temperature triaxial compression experiments. Samples with melt fractions of 0.02 ≤∅≤0.12 were deformed under water-saturated conditions at temperatures between 1373 and 1473 K and a confining pressure of 300 MPa in a gas-medium apparatus. At constant differential stress and temperature, the rate of deformation increased rapidly but systematically with increasing melt fraction. In the diffusion creep regime, at a given differential stress, samples with melt fractions of 0.02 deformed a factor of ~2 and ~20, respectively, faster than a melt-free sample. In the dislocation creep regime, a sample with a melt fraction of 0.12 deformed a factor of ~40 faster than a melt-free sample. For partially molten olivine-basalt aggregates deformed under hydrous conditions, the dependence of creep rate on melt fraction can be expressed in the form ̇e(∅)=̇e(0) exp(α∅), where α≈ 26 for diffusion creep and α≈ 31 for dislocation creep. The results of this study, combined with reasonable estimates for the spatial variation in the concentrations of water and melt (as well as for the geotherm and the activation volume for creep), provide constraints on the viscosity structure of Earth's upper mantle. As an example, we present a viscosity profile for the mantle wedge above a subducting plate, demonstrating that the viscosity in that region can vary by ~ 3 orders of magnitude over a depth of ~ 60 km due to the combined effects of water and melt weakening.