The tectonic evolution of Sulawesi is shaped by complex subduction processes, yet the geometry and extent of its slabs remain debated. Using teleseismic P-wave tomography, we present a new 3-D model of the lithosphere and underlying upper mantle beneath Sulawesi, based on passive seismic data from 89 seismic stations. Our results reveal at least three distinct slabs beneath North Sulawesi: a south-dipping Celebes Sea slab, a westward-subducting Sangihe slab, and a north-dipping structure that likely represents the Sula slab. Beyond the subduction system, we identify a prominent low-velocity anomaly beneath the East Arm of Sulawesi, extending from ~200 to ~500 km depth, which we interpret as partial melting in the mantle transition zone. This process may be driving localized volcanism in the Gulf of Tomini. These findings provide new constraints on subduction dynamics and highlight the role of deep mantle processes in driving surface magmatism.