We image the rupture of the 28 March 2005 Sumatra Mw 8.6 earthquake by back-projecting teleseismic P waves recorded by the Global Seismic Network and the Japanese Hi-net to their source. The back-projected energy suggests that the rupture started slowly, had a total duration of about 120 s, and propagated at 2.9 to 3.3 km/s from the hypocenter in two different directions: first toward the north for ~100 km and then, after a ~40 s delay, toward the southeast for ~200 km. Our images are consistent with a rupture area of ~40,000 km2, the locations of the first day of aftershocks, and the Harvard CMT Mw of 8.6, which implies an average slip of ~6 m. The earthquake is similar in its location, size, and geometry to a Mw ~8.5 event in 1861. Our estimated average slip is consistent with a partially coupled subduction interface, GPS forearc velocities, and the ~59 mm/yr convergence rate if the 2005 earthquake released elastic strain that accumulated over many hundreds of years rather than just since the last 1861 event.