Combining long-period magnetotelluric data from the spatially uniform EarthScope USArray and higher-resolution profiles, we obtain a regional three-dimensional electrical resistivity model in the Snake River Plain and Yellowstone areas (Idaho and Wyoming, United States), and provide new constraints on the large-scale distribution of melt and fluids beneath the Yellowstone hotspot track. Contrary to what would be expected from standard mantle plume models, the electromagnetic data suggest that there is little or no melt in the lower crust and upper mantle directly beneath Yellowstone caldera. Instead, low mantle resistivities (10 Ωm and below), which we infer to result from 1%–3% partial melt, are found 40–80 km beneath the eastern Snake River Plain, extending at least 200 km southwest of the caldera, beneath the area of modern basaltic magmatism. The reduced resistivities extend upward into the mid-crust primarily around the edges of the Snake River Plain, suggesting upward migration of melt and/or fluid is concentrated in these areas. The anomaly also shallows toward Yellowstone, where higher temperatures enhance permeability and allow melts to ascend into the crust. The top of the conductive layer is at its shallowest, in the upper crust, directly beneath the modern Yellowstone supervolcano.