Abstract Cratonic margins are commonly modified, yet the processes responsible remain debated. The Korean Peninsula, situated between adjacent cratons, the Japan Trench, and a back‐arc system, provides a natural setting to investigate this problem. Using 4 years of dense seismic observations, we image upper‐mantle structure beneath the peninsula with P‐wave tomography that incorporates a prescribed anisotropy model to reduce velocity artifacts. The results reveal a pronounced east–west contrast, with slow velocities beneath the eastern peninsula and fast velocities beneath the western peninsula. The eastern low‐velocity anomalies are consistent with hot and/or compositionally modified mantle, whereas the fast western anomalies may indicate ongoing lithospheric modification beneath the peninsula.