Subduction termination remains poorly understood. Sabah, Malaysia (in northern Borneo) is a prime study location due to its subduction-rich history. We investigate the impact of subduction on Sabah’s mantle transition zone via P-to-S receiver function imaging. Receiver functions are computed using high resolution passive seismic data from the northern Borneo Orogeny Seismic Survey (nBOSS), MetMalaysia and KalNet arrays. Stacking of ~1800 receiver functions reveals considerable complexity, including a low amplitude 410 km discontinuity in north-western Sabah and two distinct discontinuities at depths ~675 km and ~735 km in eastern Sabah. Since 2019, three deep (>600 km) earthquakes have occurred near these discontinuities. We attribute these observations to subducted Proto-South China Sea slab material settling at the base of the mantle transition zone, introducing cool, hydrous oceanic material with a high-basalt fraction to the mantle, thus providing an interface for deep seismicity, and facilitating garnet phase transitions at 600-700 km depth.