Traveling ionospheric disturbances generated by an epicentral ground/sea surface motion, ionospheric disturbances associated with Rayleigh‐waves as well as post‐seismic 4‐minute monoperiodic atmospheric resonances and other‐period atmospheric oscillations have been observed in large earthquakes. In addition, a giant tsunami after the subduction earthquake produces an ionospheric hole which is widely a sudden depletion of ionospheric total electron content (TEC) in the hundred kilometer scale and lasts for a few tens of minutes over the tsunami source area. The tsunamigenic ionospheric hole detected by the TEC measurement with Global Position System (GPS) was found in the 2011 M9.0 off the Pacific coast of Tohoku, the 2010 M8.8 Chile, and the 2004 M9.1 Sumatra earthquakes. This occurs because plasma is descending at the lower thermosphere where the recombination of ions and electrons is high through the meter‐scale downwelling of sea surface at the tsunami source area, and is highly depleted due to the chemical processes. Key Points A large depletion of ionospheric plasma appears above tsunami source area A tsunamigenic ionospheric hole occurs only in the large subduction earthquake A dense GPS network will provide the whole of the initial tsunami