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We obtained a 3D image of crust and mantle seismic velocity gradients beneath the state of Alaska using common‐conversion point (CCP) stacking of S‐to‐p converted body waves recorded by hundreds of stations from the NSF EarthScope Transportable Array and other portable arrays and permanent networks. Moho depths delineate the thick crust of the underthrust Yakutat terrane and the crustal root beneath the Brooks Range. The North American lithosphere is particularly thin close to the subducting lithosphere in the Alaska subduction zone, consistent with thinning of the upper plate by subduction zone flow and melt rising from the mantle wedge. The lithosphere remains relatively thin far to the northwest and north, including the Seward Peninsula and regions to the south of the Brooks Range where lithospheric extension and foundering may have played a role. The lithosphere dramatically thickens beneath the Brooks Range and northern Arctic Alaska terrane where it appears to be both cold and highly viscous. The CCP stack also revealed a pronounced positive velocity gradient at depths of 130–230 km that represents the base of a layer within the asthenosphere whose low velocities are best explained by the presence of partial melt. Although this gradient is present close to the subducting lithosphere, where partial melting is enabled by slab‐derived fluids, it is strongest beneath the Seward Peninsula and northeast of the Wrangell volcanic field, suggesting the presence of partial melt in the asthenosphere hundreds of kilometers away from the slab, likely due to decompression melting in upwelling asthenosphere. Plain Language Summary: We used recordings of distant earthquakes at hundreds of seismic stations to create a 3D image of the crust and mantle beneath the state of Alaska. We measured the thicknesses of the crust and the lithosphere, and the depth of a mantle feature that we interpreted as the base of a layer containing partial melt. The crust is particularly thick beneath the Brooks Range in the north, and where the plate subducting in the south is carrying the crust of the Yakutat terrane beneath the upper plate. The upper plate appears to have been thinned by subduction zone flow and partial melt rising from the mantle wedge in central Alaska, and it is much thicker in the far north. Above depths of 130–230 km, partial melt exists near the subducting plate, where slab‐derived fluids allow melting of mantle rocks. However, the effects of partial melt are largest beneath the Seward Peninsula and northeast of the Wrangell volcanic field, hundreds of kilometers away from the slab, where melting is enabled by rising mantle. Key Points: We image seismic velocity interfaces beneath the state of Alaska using common conversion point stacking of Sp converted phasesThe Moho, the lithosphere‐asthenosphere boundary, and the base of a low velocity asthenospheric layer are resolvedThe low asthenospheric velocities are consistent with the widespread presence of partial melt [ABSTRACT FROM AUTHOR]