An eikonal equation‐based tomography method is used to invert the high‐quality regional and teleseismic traveltime data recorded by 26 broadband seismic stations in northern Sumatra, following which we obtain the P and S wave velocity structures of the crust and mantle down to a depth of 800 km. The results of both P and S wave tomography clearly show that the Indo‐Australian oceanic plate continuously penetrates downward beneath northern Sumatra and the maximum penetration depth varies approximately from 400 km at the northern tip of Sumatra to about 800 km around the southern boundary of our study area. Significant slab folding or bending reported in the literature as the main feature of the subducted slab beneath northern Sumatra is not found in our results. Instead, our tomographic images demonstrate only a less curved slab that mimics the shape of the Sunda Trench and volcanic arc and generally extends over depths from 120 to 450 km. P wave tomography shows broad and pronounced low‐velocity anomalies beneath the island of Sumatra in the lower crust and uppermost mantle. Our model also reveals a slab tear approximately at 120‐km depth, which has been documented in previous studies and considered to be related to the eruption of the Toba supervolcano.