The slip distribution of the largest foreshock that occurred 2 days before the mainshock of the 2011 Tohoku earthquake is estimated by tsunami waveform inversion. The major slip region was located on the down-dip side of the hypocenter, and the slip amounts ranged from 0.6 to 1.5 m. By assuming the rigidity of 4 × 1010 N m-2, the seismic moment calculated from the slip distribution is 1.2 × 1020 N m (Mw 7.3). The slip distribution suggests that the largest foreshock did not rupture the plate interface where the dynamic rupture of the mainshock was initiated. The largest foreshock increased the Coulomb stress (1.6–4.5 bars) on the plate interface around the hypocenter of the mainshock. This indicates that the 2011 Tohoku earthquake was brought closer to failure by the largest foreshock.