The distribution of water in the Earth's interior reflects the way in which the Earth has evolved, and has an important influence on its material properties. Minerals in the transition zone of the Earth's mantle (from approximately 410 to approximately 660 km depth) have large water solubility, and hence it is thought that the transition zone might act as a water reservoir. When the water content of the transition zone exceeds a critical value, upwelling flow might result in partial melting at approximately 410 km, which would affect the distribution of certain elements in the Earth. However, the amount of water in the transition zone has remained unknown. Here we determined the effects of water and temperature on the electrical conductivity of the minerals wadsleyite and ringwoodite to infer the water content of the transition zone. We find that the electrical conductivity of these minerals depends strongly on water content but only weakly on temperature. By comparing these results with geophysically inferred conductivity, we infer that the water content in the mantle transition zone varies regionally, but that its value in the Pacific is estimated to be approximately 0.1-0.2 wt%. These values significantly exceed the estimated critical water content in the upper mantle, suggesting that partial melting may indeed occur at approximately 410 km depth, at least in this region.