Great volumes of water are carried downward into the mantle transition zone (MTZ, 410-670 km depth) by subducting slabs. If this water is later drawn upward, the resulting mantle melting may generate intraplate volcanism (IPV). Despite its importance, the amount and spatial distribution of water within the MTZ, and its impact on IPV, are poorly constrained. Here we use a series of plate tectonic reconstructions to estimate rates and positions of water injection into the MTZ by subducted slabs during the past 400 Myr. This allows us to construct maps of heterogeneous MTZ hydration, which we then compare to IPV locations since 200 Ma. We find a statistically significant correlation between wet regions of the MTZ and locations of IPV at the surface, but only if water remains stored in the MTZ for periods of 30-100 Myr after being carried there by slabs. We find that 42-68% of IPV is underlain by wet MTZ, with higher correlations associated with longer MTZ residence time, slower slab sinking rates, and longer time periods between MTZ hydration and IPV eruption. The correlation is highest during the Jurassic, when more extensive slab interaction with the MTZ caused a wider area of the MTZ to become hydrated. Parts of the MTZ near the western Pacific, southern Africa, and western Europe, have remained dry by avoiding wet slabs. Hydrous upwellings rising from the MTZ, some driven by interactions with subducting slabs, may be responsible for IPV rising from wet MTZ regions.