Mass rearrangement within the crust and lithospheric mantle and ocean water redistribution caused by great earthquakes are made visible by their coseismic gravity signature, nowadays detectable by the Gravity Recovery And Climate Experiment (GRACE) space mission. Here we present a novel procedure for estimating the principal seismic source parameters (hypocenter and moment tensor) that relies solely on space gravity data from GRACE, applied for the first time to the 2011 Tohoku earthquake. It yields a seismic source model that is consistent with a thrust earthquake and geological information of the subduction zone. It closely resembles the Global Centroid-Moment-Tensor Project solution based on the teleseismic wave inversion, although the moment magnitude is higher (9.13 ± 0.11 compared to 9.08) and the hypocenter is further offshore by about 40 km, within the oceanic plate. This procedure will become an important tool in seismology as it complements Centroid-Moment-Tensor analysis by exploiting the new gravity data from GRACE.