The rapid and reliable estimation of the magnitudes of large earthquakes is critical for determining the potential shaking damage and tsunami hazards. The primary challenge to rapidly and accurately estimating the magnitude of large earthquakes is the need to wait for the full waveform in order to calculate the source parameters. We used data of the shallow earthquakes in Japan from 2008 to 2016, recorded at 10°–60° (regional to teleseismic distances), to establish an operational method to quickly determine their magnitudes. Our results suggest that earthquake magnitudes can be estimated accurately 6–12 min after their origin times. The only time‐limiting factor on our method is the epicentral distances to the seismic stations. For the case of the 2011 great Tohoku earthquake, the magnitude was estimated as M 8.9–9.1 at 6–12 min after the origin time. Resolutions of the results were further investigated by bootstrap and jackknife tests and subarray analysis. Therefore, we propose building a system for determining the magnitude of large earthquakes in and around Japan using real‐time seismic data in China and worldwide. This will assist in disaster mitigation immediately after a damaging earthquake, especially for the purpose of tsunami evacuation and emergency rescue.