The seismic low velocity zone (LVZ) of the upper mantle is generally 6 associated with a low-viscosity asthenosphere that plays a key role for the 7 dynamics of plate tectonics 1. However, its origin remains enigmatic, some 8 authors attributing the reduction in seismic velocity to a small amount of 9 partial melt 2,3 , others invoking solid-state mechanisms near the solidus 4-6 , or 10 the effect of volatile contents 6. Observations of shear attenuation provide 11 additional constraints to unravel the origin of the LVZ 7. Here, we report the 12 discovery of partial melt within the LVZ from the simultaneous 13 interpretation of global 3D shear attenuation and velocity models. We 14 observe that partial melting down to 150-200 km depth beneath mid-ocean 15 ridges, major hotspots and back-arc regions feeds the asthenosphere. A small 16 part of this melt (<0.30%) remains trapped within the oceanic LVZ. The 17 amount of melt is related to plate velocities and increases significantly 18 between 3 and 5 cm yr-1 , similar to previous observations of mantle crystal 19 alignement underneath tectonic plates 8. Our observations suggest that by 20 reducing viscosity 9 , melt facilitates plate motion and large-scale crystal 21 alignment in the asthenosphere. Melt is absent under most of the continents. 22 Our finding results from the simultaneous analysis of two upper mantle 23 tomographic models of shear wave velocity (Vs) and attenuation (parameterised 24