Changbaishan/Paektu volcano straddles the border between the Democratic People's Republic of Korea and China. It was responsible for one of the largest eruptions in history, the "Millennium Eruption' of 946 CE. An episode of unrest between 2002 and 2005, characterized by inflation and seismicity, refocused attention on this volcano. While satellite remote sensing has provided synoptic observations, ground-based surveillance has hitherto supported only disparate analyses and geophysical interpretations on either side of the border. Here, we derive receiver functions using seismic records from both Democratic People's Republic of Korea and China. H-stacking indicates thick crust (up to 40 km) and high average crustal V P ∕V S (up to 1.93) beneath the volcano. Grid search inversions constrain a significant velocity reduction at 4-8 km depth (below sea level), and harmonic analysis suggests this dips away from the volcano, with shallowest depths centered beneath the volcano. Common conversion point migrations show that this anomaly extends ∼30 km from the volcano summit and possibly as far as neighboring volcanoes. The colocation of the velocity reduction with a zone of high-conductivity, low-velocity, and low-density material at the depth of the inflation source implicated in the 2002-2005 unrest indicates that partial melt is present directly beneath Changbaishan/Paektu, likely recharged during the episode of unrest. Our study highlights the importance of continued surveillance of the volcano and the need for further geophysical studies to constrain more fully the triggers for unrest and controls on its evolution.