Abstract Intraplate volcanism in Northeast Asia, located in a distant back‐arc setting, challenges conventional models of mantle convection and volatile cycling. Using receiver function data from over 300 broadband stations, we image the mantle transition zone (MTZ) beneath the Changbaishan/Paektu volcano. Our results show a modest 410‐km depression consistent with melt accumulation caused by slab dehydration and/or a hydrous MTZ, and a depressed 660‐km discontinuity linked to slab stagnation. Notably, lateral variations in the morphology of negative phases above the 410‐km discontinuity may reflect localized variations in upwelling rate and are associated with lateral heterogeneity of the underlying slab and localized uplift of the 660‐km discontinuity, suggesting focused upwelling through a potential slab lens/gap. These observations suggest that MTZ dehydration primarily drives regional intraplate magmatism, with potential lower‐mantle return flow locally enhancing this process, highlighting the critical role of the MTZ in controlling volatile transfer in deep subduction systems.