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Abstract The Alaska Peninsula has a long history of plate subduction with along‐arc variations in volcanic eruption styles and geochemistry. However, the sub‐arc melting processes that feed these volcanoes are unclear. The Alaska slab morphology below 200 km depth remains debated due to limited seismic data and thus low tomography resolution in this region. Here we utilize the newly available regional and teleseismic data to build 3‐D high‐resolutionV_PandV_Smodels to 660 km depth. We find that the high‐velocity Pacific Plate subducts to the bottom of the mantle transition zone (MTZ) with complex deformation and gaps. In the southwest, we observe a wide gap in the high‐velocity slab at 200–500 km depths. Toward the northeast, the slab becomes more continuous extending to the MTZ with a few holes below 200 km. We interpret these gaps as a slab tear that coincides with the subducted ancient Kula‐Pacific Ridge. We also invert for 3‐DV_PandV_P/V_Smodels to 200 km depth with higher resolution and find strong along‐strike changes in slab dehydration and sub‐arc melting, indicated by lowV_Pand highV_P/V_Sanomalies. Slab dehydration and sub‐arc melting are most extensive below the Pavlof and Shumagin segments in the southwest, becoming limited below the Chignik and Chirikof segments in the northeast, and extensive again beneath the Kodiak segment further to the northeast. We propose that the variations of slab hydration at the outer rise significantly influence slab dehydration at greater depths and further control sub‐arc melting beneath the Alaska Peninsula.