Spaceborne gravimetry data from the Gravity Recovery And Climate Experiment (GRACE) are processed using spatio‐spectral Slepian localization analysis enabling the high‐resolution detection of permanent gravity change associated with both coseismic and postseismic deformation resulting from the great 11 March 2011 Mw 9.0 Tohoku‐Oki earthquake. The GRACE observations are then used in a geophysical inversion to estimate a new slip model containing both coseismic slip and after‐slip. The GRACE estimated moment for the total slip, up to the end of July 2011 is estimated as (4.59 ± 0.49) × 1022 N m, equivalent to a composite Mw of 9.07 ± 0.65. If the moment for the Tohoku‐Oki main shock is assumed to be 3.8 × 1022 N m, the contribution from the after‐slip is estimated to be 3.0 × 1021–12.8 × 1021 N m, in good agreement with a postseismic slip model inverted from GPS data. We conclude that GRACE data provide an independent constraint to quantify co‐ and post‐seismic deformation for the Tohoku‐Oki event.