Nicaraguan volcanoes show globally high concentrations of geochemical tracers from dehydration of subducting crust, which may reflect a slab with unusually high amounts of H2O. To test this possibility, we measure seismic velocities at the top of the subducted plate and compare them with predictions for hydrated mafic rocks. Regional seismic P waves for intraslab events at 100--150 km depth show a high-frequency late arrival, apparently trapped in a low-velocity waveguide 2.5--6 km thick at the top of the downgoing plate, 14.5 ¿ 2.2% slower than surrounding mantle. The velocities can be explained by ≥5 wt % H2O in the subducted crust, 2--3 times the hydration inferred for other slabs by similar methods. This interpretation implies extensive hydration of the Cocos Plate off Nicaragua, perhaps enhanced by up-dip fluid flow within the slab at >100 km depth.