Bending of oceanic plates at subduction zones results in extension and widespread normal faulting1 in the upper, brittle part of the slab. Detailed seismic surveys at trenches reveal that this part of the oceanic plate could be pervasively hydrated for several kilometres below the crusta??mantle boundary. Similarly, heat-flow surveys indicate active fluid circulation within the slab. Yet, the mechanisms that enable fluids to percolate to such depths in spite of their natural buoyancy remain unclear. Here we use two-dimensional numerical experiments to show that stress changes induced by the bending oceanic plate produce subhydrostatic or even negative pressure gradients along normal faults, favouring downward pumping of fluids. The fluids then react with the crust and mantle surrounding the faults and are stored in the form of hydrous minerals. We suggest that this process is the dominant mechanism of deep slab hydration, although it may be locally aided by the enhancement in porosity due to prefailure dilatancy, pre-existing cracks and migrating fluid-filled cracks. Our results have implications for the transport of water into the deeper parts of the mantle, and for further clarifying the seismic anisotropy of slabs.